
Class '^4. 

Book 



COPYRIGHT DEPOSIT 



I 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



T»^^ 



% 9 ^^ c> 




Frontispiece. 
Granite peaks in the Yosemite. 



FIRST BOOK 



PHYSICAL GEOGRAPHY 



BY 




RALPH SyTARR, B.S., F.G.S.A. 

PROFESSOR OF DYNAMIC GEOLOGY AND PHYSICAL GEOGRAPHY AT 

CORNELL UNIVERSITY , 

AUTHOR OF "ECONOMIC GEOLOGY OF THE UNITED STATES" 

"elementary physical GEOGRAPHY" 

" ELEMENTARY GEOLOGY," ETC. 



THE MACMILLAN COMPANY 

LONDON: MACMILLAN & CO., Ltd. 
1898 

^^ ^^W^ -^ii rights reserved 

>)st COPY. 

1398. ?>^5^nH- 






.\^^.<..'-' 



8148 



Copyright, 1S97, 
By the MACMILLAN COMPANY. 

Copyright, 189S, 
By the MACMILLAN COMPANY. 



Set up and electrotyped June, 1897. Reprinted July, 
August, September, October, 1897; March, 1898. 



J. S. Cushino: & Co. — Berwick & Smith 
Norwood Mass. U.S.A. 






c.^ 



^ 



PREFACE 

In preparing my Elementary Physical Geography, I 
attempted to present the subject in such a way as to put 
it forward in its more modern aspect, and particularly to 
include the new physiography, or science of land form. 
An effort was made to cover the entire ground in a very 
elementary way ; but at once the difficulty arose that the 
field was so large, that even to present the subject in 
an elementary manner would require a good-sized book. 
To avoid this there were two things possible : one to 
omit some parts and curtail others ; the other to com- 
mence the consideration of some subjects with the assump- 
tion that either the scholars knew the preliminary points, 
or that the teacher could explain them. Both of these 
methods of shortening the book were followed, but even 
then it grew to a size entirely too large for those schools 
in which the subject has a minor place. 

The result has been, that although this book has met 
with a success wholly unexpected, many teachers who 
wish to give instruction in the netu physical geography^ 
are unable to make use of it. It is partly in the hope of 
meeting the needs of these, that I have undertaken the prep- 
aration of this smaller book, from which still other sub- 
jects are omitted, but in which the attempt is made to start 
from the beginning, and make every topic thoroughly clear, 
assuming no more than is absolutely necessary in a subject 



vm PREFACE 

which is based in part upon certain well-known principles 
of other sciences, notably physics and geology. 

If I have been successful in this effort, the, other object 
that I have in mind may also be accomplished. Now, in 
many of the better schools, geography is taught first as a 
home study of observation, and this is followed by general 
geography, and this by physical geography. But the latter 
subject, as treated in most geographies, is entirely inade- 
quate, and ought to be supplemented, or better still be 
entirely replaced, by a real study of physical geography in 
the upper grade of the grammar school. Since the great 
majority of our youth never go further than the grammar 
school, I believe that they ought not to be allowed to go 
out into life without a genuine knowledge of the main 
principles of air and earth sciences. I feel this strongly, 
not merely because of the information which they gain, 
but also because of the discipline and culture which these 
sciences can impart. 

In reality, it is for the last object that I chiefly seek, for 
I believe that with this information and training, the stu- 
dent, if he enters the high school, will then be able to go 
on with much greater profit with a more thorough study 
of the earth sciences, geology and physiography. Indeed, 
I would urge, that where the general study of physical 
geography as a whole can be put into the grammar schools, 
the high schools should take up, not a more thorough study 
of the same subject, on the " concentric " plan, but a fuller 
study of a part, or better still, several parts, like meteo- 
rology, physiography, and geology. 

Although having in mind the use of this book in the 
high school, I would frankly say that I have no sympathy 
with the conditions which seem to demand it. Modern 



PREFACE ix 

education should rise above fourteen-week courses, and it 
is better to omit physical geography, giving its place to 
a more thorough study of some other science, than to keep 
it on that plane. By peculiar merits of its own it demands 
fuller study. It is my hope, therefore, that where this book 
may be used in high schools in which conditions have 
made necessary a short course, it may help to create a de- 
mand for better and fuller instruction in the subject. 

Much of the value of the new physical geography, — in- 
deed I might almost say its whole value, aside from the 
information which it imparts, — depends upon the way in 
which it is taught. To assign certain pages to be memo- 
rized, and to stop there, whether this is done in the gram- 
mar or the high school, is to fail to obtain the full results 
which can be gained from the study. Simple class-room 
experiments ; laboratory study of specimens, maps and pho- 
tographs ; observations made by students independently, 
and discussed in the class ; practice in grouping facts which 
logically lead to conclusions ; and collective study out-of- 
doors, shoukl take the place of much of the time-honored 
recitation. No specific suggestions are made here, but 
teachers will find some in my earlier book. No school is 
so unfavorably situated that opportunities for such study 
cannot be found in abundance ; no genuine teacher will 
fail to find pleasure from the results of such study; and 
no student will fail to gain great profit from it. 

Much good comes in any study when the desire is created 
for fuller information ; and still more benefit arises when 
to satisfy this desire, students are taught that there are 
places in which to look, and are given instruction how to 
find them. To make these benefits possible, I appended 
at the close of each chapter of my Elementary Physical 



X PREFACE 

Geography, a list of books of reference selected from the 
best. It is not worth while to repeat these here; but at 
the end of this book there are a few supplementary refer- 
ences, chiefly to works which have appeared since the 
former book was published. 

This book follows approximately the same order as that 
of my Elementary Physical Geography, because I believe 
that this is the best. Still some will be found who prefer 
a different order, and there is no reason why a teacher 
who prefers to commence with the land may not do so. 
As in my Elementary Physical Geography and Elementary 
Geology, I have introduced illustrations profusely, because 
it is my belief that next to nature itself, such illustrations 
are of most value. More can be told in the space occupied 
by them than in several times that much space in type, 
and it can be told more clearly. Moreover, some of them 
can be made to serve as a basis for observation study. A 
considerable majority of the illustrations are original, but 
some are copied, and some are reproduced from photographs 
taken by others. To those who have kindly allowed me 
to use these I return especial thanks. 

RALPH S. TARR. 
Ithaca, N. Y., June 1, 1897. 



CONTENTS 



Part I. Ixtroduction 
CHAPTER I. Condition of the Earth 

PAGE 

Form of the Earth 3 

The Earth a Sphere 3 

Longitude 4 

Latitude 6 

The Earth an Oblate Spheroid 6 

General Condition of the Earth 7 

Air 7 

Ocean 7 

The Solid Earth 8 

Surface of the Earth 9 

Continents and Ocean Basins ....... 9 

Mountain Irregularities ........ 12 

Minor Irregularities 13 

Movements of the Earth 13 

Rotation 13 

Revolution 15 

The Sun in the Heavens 15 

Cause of Seasons 17 

CHAPTER II. The Universe 

The Solar System 22 

The Sun 22 

The Planets ,23 

Satellites 24 

The Universe 25 

The Nebular Hypothesis 27 

Symmetry of the Solar System .27 

The Explanation . . .28 

Facts accounted for 30 

Other Nebulae . 31 

xi 



XU CONTENTS 

Part II. The Atmosphere 
CHAPTER III. General Features of the Air 

PAGE 

Importance of the Air .... o .... 32 

Composition 33 

Oxygen and Nitrogen 33 

Carbonic Acid Gas 33 

Water Vapor . . 35 

Dust Particles 38 

Height of the Atmosphere 40 

Changes in the Air 42 

CHAPTER IV. Light, Electricity, and Magnetism 

Light 

Nature of Light 43 

Reflection 44 

Absorption 47 

Selective Scattering . 48 

Refraction 48 

The Colors of Sunrise and Sunset 49 

The Rainbow 51 

Halos and Coronas 51 

Sunlight Measurements 52 

Electricity and 3Iagnetism 

Lightning 52 

Magnetism . . . . . . . . . " . .53 

CHAPTER V. Sun's Heat 

Nature of Heat . 55 

Reflection of Heat 55 

Absorption of Heat . 57 

Radiation of Heat .-57 

Conduction of Heat . . . 59 

Convection . 59 

Heat on the Land . . . = . . . . . .60 



CONTENTS xiii 

PAGE 

Warming of the Ocean 61 

Temperature of Highlands 62 

Effect of Heat on the Air 64 

Effect of Kotation 66 

Effect of Revolution . .67 

Temperature Measurement 68 

CHAPTER VI. Temperature of the Earth's Surface 

Day and Night Change 70 

Daily Range 70 

Change with the Seasons .71 

Effect of Land and Water 72 

Irregular Changes ......... 73 

Seasonal Temperature Change 74 

Seasonal Range 74 

Influence of Latitude 76 

Influence of Altitude 77 

Influence of Land and Water .77 

Climatic Zones 78 

Isothermal Lines 79 

Temperature Extremes 84 



CHAPTER VII. Winds 

Air Pressure 85 

Measurement of Air Pressure 85 

Change in Air Pressure ......... 88 

Planetary Winds 90 

Theoretical Circulation . 90 

Trade-Wind Circulation . . . . . . . .91 

Prevailing Westerlies 93 

Periodical Winds 95 

Monsoons ... 95 

Land and Sea Breezes .97 

Mountain and Valley Breezes 98 

Irregular Winds 99 

Velocity of the Wind 99 

Measurement of Winds 101 



XIV 



CONTENTS 



CHAPTER VIII. Storms 













PAGi! 


Weather Changes . . . 


. 102 


Weather Maps 

'Comparison of Weather Maps . 

Cyclonic and Anticyclonic Areas 

The Low- and High-Pressure Areas 
Origin of the High- and Low-Pressure 


Areas 








102 
104 
107 
107 
111 


Explanation of the Winds . 
Explanation of the Rain . 
Explanation of the Temperatures 
Hurricanes or Tropical Cyclones 

Time and Place of Occurrence . 


. 








112 
113 
114 
116 
116 


Characteristics .... 










117 


Explanation .... 
Storm Winds 


° 








118 
119 


Thunder Storms .... 
Tornadoes 


• 








121 
124 



CHAPTER IX. Moisture in the Atmosphere 



Vapor 126 

Instruments for Measuring Vapor . ... • • • • 129 

Dew 130 

Frost „ ... 132 

Fog o ... 133 

Haze 134 

Mist ... o . o ....... 135 

Clouds ....... 135 

Cloud Materials , . . .135 

Forms of Clouds 136 

Causes of Clouds 139 

Rain 140 

Hail . 141 

Snow 141 

Measurement of Rainfall 144 

Nature of Rainfall 144 

Distribution of Rain . 145 

Distribution of Snowfall 148 



CONTENTS XV 



CHAPTER X. Climate 

PAGE 

Meaning of the Word Climate 149 

Climatic Zones 149 

Climate of the Tropical Zone . . 150 

Belt of Calms 150 

The Trade- Wind Belt 152 

The Indian Climate 154 

Climates of the Frigid Zones 155 

The South Frigid Zone , . . . . ... . .155 

Near the Arctic Circle 155 

In the Higher Latitudes 156 

Climates of the Temperate Zone 160 

Various Types 160 

United States Climates 161 

Difference between United States and Europe .... 162 

Variation with Altitude 163 

Differences between Ocean and Land 163 



CHAPTER XL Distribution of Animals and Plants 

Zones of Life 165 

Life in the Ocean 
Plants . 165 

167 
167 
169 
171 



Animals 

Faunas of the Coast Line (Littoral Faunas) 
Animals of the Ocean Bottom (Abyssal Fauna) 
Life at the Surface (Pelagic Faunas) . 

Life in Fresh Water 173 

Life on the Land 

Plants 174 

Animals . 178 

Distribution of Man 180 

Modes of Distribution of Animals and Plants 182 

Barriers to the Spread of Life 185 



XVI CONTENTS 

Part III. The Ocean 
CHAPTER XII. General Description of the Ocean 

PAGE 

Area of the Ocean . . 187 

Importance of the Ocean 187 

The Ocean Water is Salt 188 

Temperature of the Ocean Surface 189 

Life on the Bottom 191 

Methods used in Studying the Ocean Bed 193 

Ocean Bottom Temperatures . . . ' 195 

The Depth of the Sea 198 

Topography of the Ocean Bottom 200 

The Ocean Bed 203 

Globigerina Ooze . 203 

Red Clay 204 

CHAPTER XIII. The Movements of the Ocean 

Wind Waves .205 

The Tides 210 

Nature of the Tides 210 

Causes of Tides . . .211 

Effects of the Tides . . .213 

Ocean Currents o . . . . 215 

Differences in Temperature ........ 215 

Atlantic Currents 216 

The Explanation 218 

Effects 219 



Part IV. The Land 
CHAPTER XIV. The Earth's Crust 

Condition of the Crust . 220 

Minerals of the Crust 221 

Elements 221 

Definition 222 



CONTENTS xvii 

Minerals of the Crust : 

Quartz 222 

Feldspar 223 

Calcite 224 

Rocks of the Crust . .226 

Igneous Rocks 226 

Sedimentary Rocks 228 

Metamorphic Rocks 231 

Position of the Rocks 232 

Movements of the Crust . . „ 234 

Age of the Earth , . 236 

Geological Ages 238 

CHAPTER XV. The Wearing Away of the Land 

Entrance of Water into the Earth 240 

Return of Underground Water to the Surface 242 

Springs 242 

Artesian Wells 243 

Mineral Springs . . . 244 

Limestone Caves 246 

Breaking Up of the Rocks 248 

Methods Employed . . . . . , „ . , 248 

Difference in Result . . . 251 

Effects of Weathering 254 

Erosion of the Land . . , 257 

Destruction of the Land 260 

CHAPTER XVL River Valleys, including Waterfalls 
AND Lakes 

Characteristics of River Valleys 261 

The River Work . 264 

History of River Valleys 268 

Accidents interfering with Valley Development .... 274 

The River Course 278 

River Deltas 283 

River Floodplains 286 

Waterfalls 288 

Lakes 291 



XVlll CONTENTS 



CHAPTER XVII. Glaciers and the Glacial Period 

PAGE 

Valley Glaciers . 294 

The Greenland Glacier 300 

Icebergs 304 

Glacial Period 305 

Evidence of this . , 305 

Cause of the Glacial Period 308 

Glacial Deposits . 309 

Effects of the Glacier 310 

CHAPTER XVIII. Sea and Lake Shores 

Difference between Lake and Sea Shores 313 

Form of the Coast 313 

Sea Cliffs 314 

The Beach 317 

Wave-carved Shores 320 

A Sinking Coast 321 

A Rising Coast .323 

Marshes 323 

Coral Reefs 324 

Islands 326 

By Construction 326 

By Destruction . 328 

Promontories 330 

Changes in Coast Line 331 

CHAPTER XIX. Plains, Plateaus, and Mountains 

Plains . 332 

Plateaus 334 

Treeless Plains 335 

Mountains 

Nature of Mountains 335 

Development of a Mountain System 337 

The Destruction of Mountains 340 

Other Kinds of Mountains 342 

The Cause of Mountains 342 



CONTENTS xix 



CHAPTER XX. Volcanoes, Earthquakes, and Geysers 

Volcanoes page 

Birth of a Volcano 344 

Vesuvius = ... . . 345 

Krakatoa . . .347 

The Hawaiian Volcanoes . . 349 

Other Volcanoes 351 

Materials Erupted 351 

Form of the Cone 352 

Extinct Volcanoes 353 

Distribution of Volcanoes 355 

Cause of Volcanoes 356 

Explanation of the Differences in Volcanoes 356 

Earthquakes 357 

Geysers 361 



ILLUSTEATIONS 



PHOTOGRAPHS AND DIAGRAMS 

FIG. 

1. Ocean surface showing curvature of earth . 

2. Diagram to illustrate curvature of earth . 

3. Maps to illustrate latitude and longitude . 

4. Diagrammatic section from surface to interior of earth 

5. The two hemispheres 

6. Land and water hemispheres 

7. Section across South America, Atlantic Ocean, and Afr 

8. Globe to illustrate day and night at Equinox 

9. Globe illustrating conditions in northern summer 

10. Globe illustrating conditions in northern winter 

11. Diagram to illustrate cause for seasons 

12. Diagram illustrating small amount of heat reaching earth 

13. Kelative size and distance of planets and sun 

14. Craters on the moon .... 

15. Diagram illustrating Nebular Hypothesis 

16. Andromeda Nebula .... 

17. Diagram illustrating density of air 

18. Ideal section of atmosphere 

19. Daily temperature change, summer . 

20. Thermograph 

21. Thickness of air passed through by vertical and 

22. Diurnal variation of temperature 

23. Daily temperature range for several places 

24. Influence of ocean on daily temperature range 

25. Daily range in desert and humid tropical lands 

26. Irregularities in daily temperature range . 

27. Temperature range for several days . 

28. Seasonal temperature range, several places 

29. Seasonal temperature range, several places 



oblique rays 



4 

5 
9 
10 
11 
12 
16 
18 
19 
21 
22 
24 
25 
29 
31 
40 
63 
66 
69 
70 
70 
71 
72 
73 
74 
74 
75 
76 



XXll 



ILLUSTRATIONS 



FIG. 

30. Influence of ocean on seasonal temperature range 

31. Isothermal chart for New England 

32. Diagram showing changes of pressure 

33. Aneroid barometer 

34. Diagram showing general air circulation . 

35. Ideal circulation of surface air, southern hemisphere 

36. Monsoon of Spanish peninsula .... 

37. Summer and winter monsoon, India . 

38. Effect of sea breeze on daily temperature range 

39. Disturbance of wind by surface irregularities , 

40. Pulsation of wind 

41. Anemometer 

42. Weather conditions, Jan. 7, 1893 

43. Weather conditions, Jan. 8, 1893 

44. Weather conditions, Jan. 9, 1893 

45. Paths followed by low-pressure areas, November, 1891 

46. Weather conditions, April 20, 1893 . 

47. Weather conditions, Nov. 27, 1896 

48. Weather conditions, Jan. 12, 1897 

49. Theoretical air movement in storm 

50. Theoretical air circulation in anticyclone . 

51. Tropical cyclone in India .... 

52. Change in barometer in hurricane 

53. Temperature change in cold wave and sirocco 

54. Influence of cyclone and anticyclone on temperature 

55. Temperature change during chinook, Montana . 

56. Photograph of distant thunder storm . 

57. Map showing location of thunder storms in cyclonic 

58. Tornado near St. Paul, Minn 

59. Diagram showing change in relat 

60. Psychrometer 

61. Upper surface of valley fog . 

62. Clouds on cliff in the Yosemite 

63. Cumulus clouds . 

64. Cirrus clouds 

65. Strato-cumulus clouds . 
QQ. Cirro-cumulus clouds . 

67. Photograph of large hailstones 

68. Photograph of snow flakes . 

69. TJain gauge .... 



ve humidity 



ILLUSTRATIONS 



XXlll 



FIG. 

70. Desert vegetation in the west . 

71. Midnight sun, northern Norway 

72. Ice-covered sea in the Arctic 

73. Land in Greenland, summer 

74. Greenland ice sheet .... 

75. Cold wave, March 13, 1888 

76. Map showing snowfall of United States 

77. Mangi'ove swamp, Bermuda 

78. Seaweed mat, Cape Ann, Mass. 

79. Corals, Great Barrier reef, Australia 

80. Deep-sea fish 

81. Deep-sea crinoid .... 

82. Semi-tropical forest in Florida . 

83. Cactus in Arizona desert . 

84. Mountain peak, crest of Andes, Peru 

85. Near timber line, Rocky Mountains . 

86. Arctic flora in snow .... 

87. A Bermuda road .... 

88. Bit of Bermuda landscape . 

89. Arctic sea ice 

90. Deep-sea sounding machine 

91. Deep-sea dredge on ocean bottom 

92. Temperature of ocean bottom of north Atlantic 

93. Temperature of ocean at various depths 

94. Section from Atlantic to Gulf of Mexico 

95. Section of ocean from New York to Bermuda 

96. Section across Atlantic showing temperature and 

97. Ocean bottom topography (Jones model) 

98. Ocean bottom topography (Jones model) 

99. Diagram showing approach of wave on beach 

100. Diagram illustrating origin of tidal wave . 

101. Diagram showing advance of tidal wave in Atlantic 

102. Diagram showing cause of spring and neap tides 

103. Diagram showing currents of eastern north Atlantic 

104. Stratification in horizontal rocks 

105. Quartz crystal 

106. Piece of calcite 

107. Section of diabase enlarged by microscope 

108. Diagram illustrating intrusion of granite . 

109. Consolidated pebble bed .... 



depth 



PAGE 

153 
156 
157 
158 
159 
160 
161 
166 
167 
169 
170 
171 
174 
175 
176 
177 
178 
182 
183 
190 
192 
194 
195 
196 
197 
199 
200 
201 
202 
207 
210 
211 
212 
217 
220 
222 
224 
226 
227 
229 



XXIV ILLUSTRATIONS 

FIG. PAGE 

110. Beach, Cape Ann, Mass . 230 

111. Coquina 231 

112. Diagram of volcano in cross section 233 

113. Crumpling of rock 233 

114. Diagram illustrating faults 235 

115. Photograph of small fault 236 

116. Folded rocks " 236 

117. A monocline . . . 237 

118. Section of gneiss enlarged by microscope 241 

119. Diagram illustrating conditions in hot springs ^ . . . 242 

120. Diagram illustrating cause of hillside springs .... 242 

121. Diagram illustrating artesian wells 243 

122. Diagram illustrating artesian wells 244 

123. Hot Springs, Yellowstone 245 

124. Howe's Cave, New York 246 

125. Natural Bridge 247 

126. Column in cavern 248 

127. Effect of frost action on mountain top 249 

128. Effect of weathering in arid lands 252 

129. Butte in western Texas 253 

130. Crumbling of rocks on mountain side . . . . . 255 

131. Decaying granite, Maryland . 256 

132. Diagram illustrating residual soil 257 

133. Bad Lands, South Dakota .258 

134. River gorge in Peruvian Andes 262 

135. Rocky stream bed in Adirondacks 264 

136. Enfield Gorge, Ithaca, N.Y 265 

137. Diagram illustrating stream action and weathering . . . 267 

138. Meandering of Missouri River 268 

139. Young valley, central New York 269 

140. Diagram illustrating base level 270 

141. Diagram illustrating development of stream valley . . .271 

142. Broad mature valley, Ithaca, N.Y 271 

143. Delaware Water Gap . . . . . . . .272 

144. Cross section of Colorado River 274 

145. View in Colorado canon 274 

146. Diagram illustrating Chesapeake Bay river system . . . 275 

147. Drainage in mountain 279 

148. Drainage on plain . 279 

149. Interlocking tributaries 280 



ILLUSTRATIONS 



XXV 



FIG. 

150. Changing of mountain tops to valleys 

151. River flowing in anticline . 

152. Cross section of delta . 

153. Alluvial fans in the west . 

154. Waterfall, central New York 

155. General view of Niagara Falls 

156. Peat bogs in Adirondacks . 

157. Snowfield in high Alps 

158. An Alpine glacier 

159. Crevassed surface of Muir glacier, Alaska 

160. Margin of Cornell glacier, Greenland 

161. Delta in glacier lake .... 

162. Scratched glacial pebble, Greenland . 

163. Ice floating in water .... 

164. Iceberg off North Greenland coast , 

165. Map of United States showing extension of 

166. Boulder clay. Cape Ann, Mass. 

167. Glaciated rock surface in Iowa . 

168. Terminal moraine hills, Ithaca, N.Y 

169. Boulder-strewn moraine, Cape Ann, Mass 

170. Sea cliff, Bermuda 

171. Undercut sea cliffs, Bermuda 

172. Wave-cut cliff, Lake Superior 

173. Bar across bay, Cape Breton, Nova Scotia 

174. Bars on shore of Martha's Vineyard 

175. Tiny wave-carved bay, Cape Ann, Mass. 

176. Depressed coast of part of Connecticut 

177. Beach and coral reef, coast of Florida 

178. Serpula atolls, Bermuda . 

179. Wave-cut islands, shore of Bermuda 

180. Islands caused by sinking of Bermuda 

181. Island joined to land by bars 

182. Plain of Everglades, southern Florida 

183. Diagram illustrating dissection of plain 

184. Sections of Appalachian Mountains . 

185. Grandfather Mountain, North Carolina 

186. Mount Moran, Teton Mountains 

187. Near timber line, Gallatin Mountains, Montana 

188. Pompeii and Vesuvius 

189. Mauna Loa, Hawaiian Islands 



ice in glacial period 



PAGE 

281 
282 
283 
285 
288 
289 
290 
295 
296 
297 
301 
302 
302 
303 
304 
305 
306 
307 
308 
309 
315 
316 
318 
319 
320 
321 
322 
325 
327 
328 
329 
330 
332 
334 
336 
337 
339 
341 
347 
348 



XXVI 



ILLUSTRATIONS 



FIG. PAGE 

190. Crater of Kilauea 349 ' 

191. Tiny volcano, Mediterranean 350 

192. Popocatapetl, Mexico . . 353 

193. Volcanic necks, New Mexico 354 

194. Distribution of Volcanoes 355 

195. Effect of Japanese earthquake, 1891 358 

190. Diagram illustrating earthquake wave 359 

197. Isoseismals, Charleston earthquake 360 

198. Giant Geyser, Yellowstone 361 



PLATES 



Granite peaks in the Yosemite . 

1. Isothermal chart of the world for July 

2. Isothermal chart of the world for January . 

3. Isothermal chart of the world for the year . 

4. Isothermal chart of the United States for July 

5. Isothermal chart of the United States for January 

6. Isothermal chart of the United States for the year 

7. Chart showing isobaric lines for the world . 

8. Map showing prevailing winds of the globe for July 

9. Map showing prevailing winds of the globe for January 

10. A West Indian hurricane 

1 1 . Typical winter storm 

12. Rainfall chart of world 

13. Rainfall chart of United States .... 

14. Average temperature of sea surface . 

15. Depth of Atlantic Ocean 

16. Chart of ocean currents 

17. Three rock specimens (diabase, granite, and gneiss) 

18. Delta of Mississippi 

19. Map of drowned coast, Maine .... 

20. Mountain ridge in the northwest 



PACING PAGE 

Frontispiece 



79 

80 

81 

82 

83 

84 

90 

92 

93 

116 

117 

146 

147 

189 

198 

214 

227 

283 

314 

336 



ILLUSTRATIONS xxvii 



ACKNOWLEDGMENT OF ILLUSTRATIONS 

Aside from those that are original, illustrations in this book have been 
obtained from the following sources. Some of these have been more or 
less modified to suit the needs of tlie book. A very few that have been 
borrowed are not acknowledged because the original source is not known. 
I am also indebted to Mr. B. F. White and Mr. J. O. Martin for some of 
the photographs. 

Abbe, Annual Report, Signal Service, Part 2, 1889, Figs. 18 and 39. 

Agassiz, Three Cruises of the Blake, Figs. 80, 81, 92, 93. 

Bailey, Prof. L. H. (Photographs by), 82, 177, 182. 

Ball, Popular Astronomy, Fig. 11. 

Blanford, Climates and Weather of India, etc., Figs. 37 and 51. 

Buchan, Challenger Reports, Atmospheric Circulation, Plates 1, 2, 3, 7, 

8, and 9. 
Chamberlin, Third Annual Report, U. S. G. S., Fig. 165. 
Dutton, Sixth Annual Report, U. S. G. S., Fig. 193 ; same, Ninth Annual 

Report, Fig. 197. 
Davis, Series of Lantern Slides for Schools, Fig. 143. 
Freiz, J. P. (Dealer in Meteorological Instruments), Baltimore, Md., 

Figs. 20, 33, 41, 60, 69. 
Gardner, J. L., 2d (Photographs by), Figs. 166 and 169. 
Gilbert, Second Annual Report, U. S. G. S., Fig. 153; same, Fifth 

Annual Report, Fig. 172. 
Hann, Berghaus Atlas der Meteorologie, Plate 12, modified. 
Harvard College Astronomical Observatory Annals, Vol. XXXI., Fig. 31. 
Hayden, West Indian Hurricanes, etc.. Fig. 75. 

Haynes, F. Jay (Photographer), St. Paul, Minn., Figs. 123, 159, and 198. 
Hellmann, Schneekrystalle, Fig. 68. 
Howes, C. H. (Photographer), Ithaca, N.Y., Fig. 139. 
Jackson Photograph Co., Denver, Col., Figs. 62, 85, 126, 145, 155, 186, 

and 192. 
Johnston-Lavis, South Italian Volcanoes, Fig. 191. 
Jones, Thomas, Chicago, 111. , Photograph of copyrighted globe. Figs. 97 

and 98. 
Kent, Great Barrier Reef, Fig. 79. 
Keyes, Fifteenth Annual Report, U. S. G. S., Fig. 131, Vol. III.; Iowa 

Geological Survey, Fig. 167. 



XXVlll ILLUSTRATIONS 

Koester (Photographs by, sold by Fredrick and Koester, St. Paul, 

Minn.). Printed in Am. Met. Jour., VII., 1891, Fig. 58. 
Langley, American Journal Science, Vol. XLVII. , 1890, Fig. 40. 
Libbey, Prof. W., Jr. (Photographs by), Figs. 86, 189, and 190. 
McGillivray (Photographer), Ithaca, N.Y., Fig. 136. 
Murray, Challenger Reports, Final Summary, Plates 14 and 15. 
Nasmyth and Carpenter, The Moon, Fig. 14. 
New York State Weather Bureau (from records of), Figs. 19, 22, 26, 27, 

32, 52, 53, 54, and 59. 
Notman (Photographer), Montreal, Canada, Plate 20. 
Sigsbee, Deep Sea Sounding and Dredging, Fig. 90. 
Steeruwitz, First Annual Report, Texas Geological Survey, Fig. 129. 
Stoddard, S. R. (Photographer), Glens Falls, N.Y., Figs. 124, 135, and 

156. 
Thomson, Challenger Reports (Narrative), Fig. 91. 
Thornton, Advanced Physiography (from a Photograph by Mr. Roberts), 

Fig. 16. 
United States Coast Survey (Maps of), Plates 18 and 19, modified. 
United States Geological Survey (Maps of). Figs. 138, 147, 174, and 

176, modified. 
United States Geological Survey (Photographs by), Figs. 70, 113, 125, 

185, and 187, and Frontispiece. 
United States Geological Survey Folios (Campbell), Fig. 184 (Hayes), 

Fig. 151. 
United States Weather Bureau (based upon maps and records of) , Plates 

4, 5, 6, 10, 11, and 13, and Figs. 42, 43, 44, 45, 46, 47, 48, 55, 57, and 

76. 
Ward, Set of Cloud Slides (Riggenbach, Burnham, etc.). Figs. 56, 61, 

63, 64, 65, and 66. 
Williston, Prof. S. W., Lawrence, Kansas (Photographs by). Figs. 128 

and 133. 



OHIO SUPPLEMENT. 



Geology and Topography. — The rocks of Ohio are all 
sedimentary (p. 228), having been deposited in the sea at 
a time when there was a great ocean extending over the 
Central States. This was during the Paleozoic time (p, 239), 
and the strata belong to the Silurian, Devonian, and Car- 
boniferous periods. In this sea, where now the state of 
Ohio stands, great sheets of shale, limestone, and sandstone 
were spread out over the ocean bottom (p. 233). 

In many of these rocks proof of this ocean origin may be 
found in the fossils (p. 238) that are entombed in them. 
Each fossil represents the remnants of an animal or plant 
which lived in this sea, and, settling to the bottom, became 
buried in the sandy, muddy, or limy beds. By the deposit 
of cement (p. 230) these soft beds have become consolidated 
to form hard rock, and the fossils have been preserved in 
them. 

These layers of stratified rock were laid down horizontally, 
and, now that they are raised above the sea, they are still in 
a nearly horizontal position. You can see that this is so in 
any ledge or quarry in the state. The first part of this sea 
bottom to be raised above the surface was a broad tract in 
the southwestern part of Ohio. It was left as a great and 
low dome extending across the boundary of Ohio into Ken- 
tucky and Indiana, and has been called the Cincinnati Arch. 
For a long time it stood as a low island in the Paleozoic sea. 

1 



2 PHYSICAL GEOGEAPHY. 

Then, toward the close of the Paleozoic, in the Carboni- 
ferous period, the depth of the sea in the eastern part became 
less, and a great shallow sea extended from this part of the 
state to the very base of the Appalachian Mountains in West 
Virginia and Pennsylvania. Indeed, it was then that the 
Appalachians were being formed ; and, as they rose in great 
folds of rock (p. 336), the region to the west of them, where 
Ohio now stands, was lifted also, but, in this part, without 
any folding or crumpling of the rocks, such as occurred 
where the mountains rose. 

Sometimes this shallow sea bottom was lifted above the 
water, and great swampy plains, perhaps somewhat like those 
of Florida (p. 333), were formed. On these swamps the 
coal jDlants grew and built beds of swamp muck, which were 
later buried beneath layers of clay and sand, and slowly 
changed to the coal which is so valuable to the states of 
Ohio, Pennsylvania, and West Virginia. 

Finally Ohio became dry land, and so it has remained 
nearly, if not all of the time since. This all happened so 
long ago that there has been much chance for change. While 
the state has been standing weathering in the air, as a dry 
land part of the continent, and rivers have been slowly carry- 
ing the rocks away. The streams have cut down through 
the sheets of sedimentary rock and carried a great deal off 
to the sea, including many hundreds of thousands of tons of 
coal. These rivers have been so long at work that they how 
occupy mature valleys (p. 270), excepting in those places 
where they have been locally changed or rejuvenated (p. 274) 
by the glacial deposit to be described later. 

The topography of Ohio is therefore that of a plain, un- 
derlaid by nearly horizontal rocks, and cut by many deep 
and broad valleys with gently sloping sides. Standing in 
the bottom of some of these valleys, such as the Ohio, with 



OHIO SUPPLEMENT. 8 

the hills rismg from 400 to 700 feet above the valley bottom, 
one might not recognize the scenery as that of a plain ; bnt 
if one should go to the top of a high hill, he would find that 
many other hills rise to nearly the same level, and that they 
all form a part of a dissected plain. In fact, if one could 
look down upon Ohio from above, he would see that it was 
really one great plain, higher along the Ohio divide, which 
extends in a northeast and southwest direction across the 
state, and where some of the highest hills are 1200 to 1500 
feet above the sea level. Moreover, this extensive plain is 
but a small part of a much larger one extending from the 
Appalachians to the Rockies, and rising higher still near the 
base of these mountains. This plain is really a broad pla- 
teau near the base of the Rockies, and also near the base of 
the Appalachians, and its lowest part is near the middle, 
where the Mississippi River flows. 

Through this dissected plain many streams extend, most 
of them entering the Ohio, either directly or through the 
Wabash, but some passing into the St. Lawrence drainage 
through Lake Erie. The drainage lines of the state may 
be studied on any good map of Ohio. 

Effects of the Glacial Period. — In the period just preced- 
ing the present, that is, during the Pleistocene (p. 239), 
there came down over this region of plains the great conti- 
nental glacier (pp. 305-312), and this changed the physical 
geography of Ohio in a most important manner. The glacier 
did not cover all of the state, but left the southeastern corner 
uncovered (Fig. 199). In other parts of Ohio proof that 
the ice visited the region may often be found, for there are 
pebbles frequently scratched (Fig. 162) and quite different 
from the rocks near by ; and, where the bare rock has been 
recently uncovered, glacial stride (Fig. 167) may be found, 
showing where the glacier has ploughed over the bed rock. 



4 PHYSICAL GEOGRAPHY. 

These strite tell the direction from which the ice came, and 
the arrows on the map (Fig. 199) show that this direction 
was mainly from the north and northeast. But in different 
parts of the state there were currents moving in various 
directions, as will be seen by studying the map (Fig. 199). 




Fig. 199. 

Map showing moraines of Ohio by dots; direction of ice movement by arrows : 
and beach lines by heavy and dotted lines southwest of Lake Erie. After 
Leverett. 



Along the margin of the glacier a terminal moraine (p. 308) 
was built, and when the glacier slowly melted away, so that 
its front stood at different points north of this termijial 
moraine, other morainal deposits, known as moraines of re- 
cession, were built. The dotted spaces on Fig. 199 show 
Avhere these moraines of recession were built, and each line 
represents a halt in the withdrawal of the ice. Many of the 
small drift hills of Ohio are parts of this moraine, and per- 
haps some of them are so near that you yourself may see 
them. 



OHIO SUPPLEMENT. 5 

Not only were moraines built at the margin, but a sheet of 
till (p. 306) was deposited wherever the ice stood. This till 
forms the soil of much of the glaciated portion of the state, 
and in some places, particularly in the western part of Ohio, 
it is very thick. Some of the wells that have been bored for 
oil have passed through 200, 300, 400, and even 500 feet of 
glacial drift. In many places this sheet of glacial deposit is 




Fig. 200. 
Till plain near Columbus, Ohio. 



SO deep that the river valleys liave been made shallower, and 
some entirely buried beneath the drift. As a result, there 
are buried valleys in places where no one would know about 
it, if wells had not been sunk there, showing hills and valley 
bottoms beneath the glacial deposits. The Cuyahoga vallej^ 
is one of those tliat is deeply filled with drift. 

The moraines and the till sheet have been laid down irreg- 
ularly, so that little lake basins have been formed ; and the 
many ponds and tiny lakes, found here and there in the 
state, have been caused in this way. Many of tlie swamps 



6 



PHYSICAL GEOGRAPHY. 



and peat bogs are ponds of this kind that have been filled 
by the aid of vegetation (p. 292). Even Lake Erie itself 
has been partly caused by deposits of glacial drift which 
have choked up the valley that, before the glacial period, 
was occupied by a river where Lake Erie is now situated. 
Probably, also, glacial erosion (p. 312) has helped to deepen 
the basin of Lake Erie, by scooping out some of the rock as 
the ice passed along the Erie valley. 

By the deposit of all this drift, many other changes wei'e 
made in the drainage. The gorges and waterfalls that 
occur in the state are the result of some of these. Where 

these young portions of rivers 
are found, it may be assumed 
that the streams are not flowing 
in their old preglacial valleys, 
but that they have cut out new 
valleys since the glacial period. 
One of the most important 
effects of the glacier was to 
rob the St. Lawrence system of 
many of its tributaries and give 
them to the Ohio. Li Fig. 201 
we have a sketch map showing 
the present drainage, and in 
Fig. 202 a similar map showing 
the probable course of the 
streams before the glacial 
period. By comparing these 
maps, it is seen that the entire 
upper headwaters of the Ohio 
were apparently given to it 
during the glacial period. What a vast difference it would 
have made to the state had this not been done ! The Ohio 



1 

PRESENT ^ 
DRAINAGE SYSTEMS ^\^ 




^m 


P^^ 


& 

H 


%S}s£^^'-^-Sy 


>•:;-- 


v^" /af!/ 




x /^ 





Fig. 201. 

Present drainage lines of Upper Ohio 

Chamberlin and Leverett. 



OHIO SUPPLEMENT. 



.7 



PROBABLE 

PREGLACIAL DRAINAGE 

OF THE 

UPPER OHIO REGION 



would have been so much smaller than now that it could not 
have been nearly so useful as it is. 

One of the reasons why it is believed that all these changes 
occurred is that in some places (as near the mouth of the 
Grand River) the old valleys are deeply filled with drift. 
A second reason is that the Ohio narrows up near North 
Martinsville so that this seems 
to be a divide, over wliich the 
Avater poured when the ice was 
here, and cut the valley so low 
that, after the ice had gone, 
the drainage was able to flow 
southward instead of north- 
ward, as it formerly had. A 
third reason for this belief is 
that the rock floor of the val- 
leys, as revealed by borings, 
indicates a northward flow for 
some of the Ohio headwaters. 

There are many other inter- 
esting effects of the glacier on 
the drainage of the state. For 
instance, notice on Fig. 199 
how peculiarly St. Joseph's 
and St. Mary's rivers unite. 
They flow together, one from 
the southeast, and one from 
the northwest, as if they were going toward the Wabash; 
but, instead of doing this, they unite and flow back toward 
Lake Erie through the Maumee. Together the three streams 
form something like the barbed head of an arrow. The 
reason for this, as you can see on the map (Fig. 199), is that 
the two rivers flow on the Avestern side of a moraine, which 




Fig. 202. 

Probable preglacial drainage of Up- 
per Ohio region. Chamberlin and 
Leverett. 



8 



PHYSICAL GEOGRAPHY. 



prevents them from sooner turning toward the east. It 
will be noticed that several other streams have their courses 
determined by the moraines. 

When the ice was withdrawing from this section, a tongue, 
or lohe^ extended up the Maumee valley, causing the moraine 
to turn southwest in that valley ; but in time the ice front 
began to withdraw from the Maumee. This river naturally 

flows northeastward, and con- 
sequently the ice formed a 
dam across the river (Fig. 
203) so that the Maumee 
could not flow into Lake 
Erie as it now does (p. 292). 
This caused a lake to form, 
which had an overflow past 
Fort Wayne, Indiana, into 
the Wabash. The shores of 
this lake can now be plainly 
seen in the Maumee valley at 
an elevation of about 250 feet 
above the lake and along the 
line marked on Fig. 199. 
It 23assed through Findlay, 
Delphos, and Bryan. Where 
the overflow occurred, near 
Fort Wayne, there is a broad channel cut in the earth, 
which is now not occupied by a stream. 

As the ice withdrew still further from this region, it 
opened a still lower outlet for these waters past the present 
city of Chicago, and then the Maumee lake fell to a lower 
level (Fig. 204), and while the water stood here another 
beach was built. It passes through Tiflin, Belmore, and 
Delta (Fig. 199). This beach extends eastward along the 




Fig. 203. 

Map showing outline of glacier front 
and. position of glacial lakes while 
the ice was receeding. Present lakes 
shown by dotted lines. Taylor in 
Dryer's Indiana Studies. 



OHIO SUPPLEMENT, 



9 



shores of Lake Erie, passing through the city of Cleveland ; 
and there are still lower beaches between these and the lake, 
marking drops in the level of the water as lower outlets 
were discovered, until, finally, the outflow was eastward 
when the ice front had melted back far enough to the 
north to allow the waters to flow in that direction. 







Fig. 204. 

Same as Fig. 203, later stage when the outflow was past Chicago. Taylor in 
Dryer's Indiana Studies. 



Not only are there beaches of sand and gravel along these 
lines, but also extensive deposits of clay in the Maumee 
valley and along the Erie shore. These were clays de- 
posited in the glacial lakes along the ice front, somewhat 
as clay is now being carried into Lake Erie by the various 
streams and deposited over its floor. In this clay-covered 
section of Ohio there were extensive prairies (p. 335) when 
the state was discovered ; but in some of the other parts of 
the state forests existed, though most of tliem have now been 
removed because of the value of the land for farming. 

Industries of the State. — A region owes its industries and 
material development to its physiography; and even the 
people themselves are influenced by it. Few states furnish 



10 PHYSICAL GEOGRAPHY. 

a better illustration of this influence than Ohio. The climate 
is pleasant and temperate and the rainfall heavy enough, and 
generally uniform enough, to ensure good crops. This rain 
falls upon plains, which, even where most dissected, are only 
cut into hills and valleys of moderate slope. This, surface is 
for the most part covered with a rich soil, and over the larger 
part of the state by a soil brought by the glacier. In places 
where a soil has resulted from the decay of sandy rocks 
(p. 256) it is liable to be sandy and sterile; but when the 
soil has been transported, as in most of Ohio, even a sandy 
rock may be covered by a blanket of rich soil. Because of 
the level surface, the fertile soil, and the rainfall, Ohio is 
eminently adapted to successful agriculture. So it is that 
the state is so productive of the various farm crops, dairy 
products, and wool. 

Besides the general conditions of climate, there are many 
local modifications, as, for instance, along the shores of Lake 
Erie, where the climate is made more moderate by the pres- 
ence of the water, so that fruit raising is one of the indus- 
tries there. In the southwestern part of the state, fruit, 
tobacco, and other delicate crops thrive well because of the 
more southern latitude ; and oftentimes there is a difference 
in effect of climate between hill top and valley bottom. 

Beneath the soil are other natural resources. The coal 
beds furnish abundant fuel ; petroleum and natural gas ^ 
furnish heat and light ; and the iron beds have been of 

1 These substances are hydrocarbons, one liquid, the other gaseous, and 
both derived from the distillation of organic remains in the rocks, notably, 
in Ohio, in the Trenton limestone. They are formed somewhat as marsh gas 
is formed in swamps ; but, being unable to escape from the rocks, they have 
slowly accumulated there until men have bored wells to the places where 
they are stored. For an excellent account of the Ohio oil and gas, see 
Orton, in the Ohio State Geological Survey Report, Vol. VII., or Part 2, 
Eighth Annual Report, U. S. Geological Survey. 



OHIO SUPPLEMENT. 11 

importance in starting iron manufactories, though some of 
them are now maintained by iron brought from beyond the 
state. The excellent building stones found among the hori- 
zontal sedimentary strata have been a source of wealth, and 
other mineral resources have been of importance. 

With the natural resources of the state, of necessity manu- 
facturing has developed to meet the needs of the producer 
and consumer. All of these industries have been greatly 
aided by the drainage lines and the general levelness of the 
region. The great Ohio, swelled in volume by the addition 
of extensive headwaters, is navigable, and upon it, as upon 
its larger tributaries, the crops and manufactured articles 
have been shipped with ease. Many towns and cities on 
these rivers, headed by the largest city in the state, Cincin- 
nati, prove the importance of these drainage lines ; for at 
these places factories have been started, and material shipped 
in such quantities that cities have of necessity grown. 

Then, also. Lake Erie bounds northern Ohio, and the prod- 
ucts of the state may be shipped over it, while the products 
of other sections may be brought in upon the same waters. 
This opens up not only all of the immense area around 
the Great Lakes, but even the ocean itself. Along the lake 
shore manufacturing towns and shipping ports have grown, 
and these are naturally located where vessels may enter to 
load and unload their cargoes: that is, where there are har- 
bors. Toledo, on a harbor at the extreme end of the lake, 
and Sandusky, on another harbor, are illustrations of this. 
Cleveland, a city almost as large as Cincinnati, is also situ- 
ated on a natural harbor which Avas large enough for all 
purposes when the city was founded, but has long since 
become too small, so that now an artificial harbor has been 
made necessary. With the growth of the city an extensive 
breakwater has been built in order to accommodate the grow- 



12 PHYSICAL GEOGRAPHY. 

ing shipping industr}^ Each of the large cities, as well as 
most of the small ones, owes its location to some natural 
feature which gave it some superior advantage and permitted 
it to grow. A person living in such a place can easily find 
out what this was in each particular case. 

Man has of course been at Avork improving the opportuni- 
ties that nature offers. He has not merely tilled the soil, 
extracted the mineral treasures, and manufactured articles 
from these products, but he has improved the means of carry- 
ing these materials about. First of all, before railroads were 
of importance, canals were dug, and this was made possible 
in Ohio by the levelness of the surface. Over these canals 
products could be carried from places which had no natural 
waterway, and in this way also towns were caused to develop. 
The building of canals in this state has had an important 
effect upon the growth of many towns, and even large cities, 
such as Toledo, Cleveland, and Cincinnati; for the canals 
connect them with large areas of the state by a very service- 
able waterway. 

Now that railroads have been built they have served chiefly 
to make the cities previously established grow more rapidly. 
These cities got their start because of natural advantages, 
and the railroads were obliged to go to them, so that even 
the railroads themselves have been influenced by the natural 
features; and this, too, in still another direction, for if you 
will look at a map of Ohio you will see that a great many of 
the railroads follow the valleys, though in the places where 
the surface is most level it has been possible for them to 
extend across country. 

All who study this book may, if they have the desire, learn 
some interesting and valuable lessons in attempting to find 
out how far their homes, and even their very lives, have been 
influenced by the physical geography of the region in which 



OHIO SUPPLEMENT. 13 

they dwell. Many gifts have been placed before man, and 
he has not been slow in finding them and putting them to the 
uses for which they are suited. If you have learned this 
lesson, and seen its application in some cases, you have been 
rewarded for this study. 

The teacher may wish to read more upon the subject of the physical 
geography of Ohio, and for this reason these few references are appended. 
First of all are the reports of the Ohio State Geological Survey, every one 
of which contains much of value, and in some of which the county 
geology is discussed. These books are in many private and public 
libraries, and copies may often be found in the second-hand bookstores. 
Dryer's Studies in Indiana Geology (Inland Pub. Co., Terre Haute, Ind,, 
1897, $0.50) contains much on the physical geography of Ohio, especially 
on the Great Lake history. The change in the Ohio drainage, mentioned 
above, is discussed by Chamberlin and Leverett in the American Journal 
of Science, 1894, Vol. XLVII., pp. 247-283 ; and the moraine of Ohio by 
Leverett in the same journal, 1892, Vol. XLIIL, pp. 281-301. There 
are many other papers on the geology of the state, and reference to some 
of these may be found in the sources mentioned in this paragraph. 



Part I 
INTRODUCTION 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



>>@<c 



CHAPTER I 




CONDITION OF THE EARTH 

Form of the Earth : The Marth a Sphere. — Standing 
upon the seashore, and looking out upon the broad ex- 
panse of water, we 
see ships sailing 
along, some near 
at hand and some 
far away. Those 
that are near show 
the sails, spars, and 
even the hull down 
to the water's edge, 
but only the sails and masts of the more distant ones are 
seen, and perchance one in the offing is detected only by 
its topmast (Fig. 1). 

This is because the surface of the earth, and the water 
upon it, is curved (Fig. 2). The vessel gradually disap- 
pears behind the curvature of the earth, just as a man 
disappears from sight as he passes over the crest of a 
hill. There are other _pr oofs that the earth is a spherical 

3 



Fig. 1. 
The ocean surface to show curvature of the earth. 



4 FIBST BOOK OF PHYSICAL GEOGBAPHT 

body. For instance, if we should start on one of the 

ships, we might pass entirely around the globe and return 

to the point whence we started. Or, by travelling over land 

and water, we can 

A B 

C___ — : — — — — i) go due east or due 

^iG- 2. west, and in time 

To illustrate curvature of earth. A person stand- f^n^ OUr^iclveS back 
ing at B could not see an object at C unless it . . 

rose to the level of the line AB. ^t the startnig pomt 

(see a globe). 

Longitude. — Should a dozen people start from as many 
places, such as New York, San Francisco, London, etc., 
and be able to go due north, their paths would all con- 
verge toward a point at which they might eventually 
meet; and this point, which is so enwrapped in ice and 
snow that it has not yet been visited by man, is called 
the North Pole. Passing due south from these same places, 
the travellers would in time meet at a point in the south, 
which is called the South Pole^ and this region also is 
inaccessible to man.^ 

In mapping the globe, geographers are in the habit of 
projecting lines in the direction of these imaginary jour- 
neys, and these all converge toward the poles. These 
meridians., or lines of longitude, are 360 in number, and 
each of these is spoken of as a degree of longitude.'^ 

1 The North and South Poles are the imaginary points on the surface 
of the earth through which the axis of the earth emerges. This axis is 
that about which the earth rotates, just as a globe or an apple may be 
made to rotate about an axis. These are not the same as the magnetic 
poles toward which the compass needle points. The north magnetic pole 
lies to the southward of the true North Pole, and is situated in Boothia 
Land, north of Hudson's Bay. 

2 Since there are 360 lines of longitude it follows that at the equator 
the length of a degree of longitude is very nearly 69.16 miles. As 



CONDITION O^ THE EART3 5 

The distance between these varies, for they broaden out 
and spread apart as the distance from the poles increases 
(Fig. 3). Where furthest apart the distance between two 
meridians is about 69 miles. Each degree (°) is divided 
into 60 minutes ('), and each minute into 60 seconds ("), 
and the longitude of any place is given in degrees, min- 
utes, and seconds. Greenwich Observatory, England, has 
been chosen by English-speaking people as the place from 




Fig. 3. 
To illustrate latitude and longitude. 



which to start in numbering these degrees.^ From Green- 
wich as the zero, the meridians are numbered toward the 
east until 180° is reached, and this is known as east lon- 
gitude^ while west longitude is located in the same way 



we proceed toward the poles the length of a degree of longitude becomes 
less and less until it is at the poles. 

1 In France the Observatory of Paris is taken as the starting place, and 
this lies 2° 20' 9" east of Greenwich ; but English-speaking people do not 
use this. 



6 FIRST BOOK OF PHYSICAL GEOGRAPHY 

toward the west. Thus the United States is in west 
longitude. 

Latitude. — In order to locate places on a sphere, we 
must know not only the longitude, or the east or west 
distance from a place, but also the distance in a north 
or south direction from some definite part of the earth. 
Therefore a series of imaginary circles are passed around 
the earth in an east and west direction. In numbering 
these, the zero circle is placed midway between the two 
poles, and to this the name Equator is applied. The space 
between each pole and the Equator is divided into 90°, 
the length of a degree of latitude varying somewhat, but 
being about 69 miles. These degrees are also divided into 
minutes and seconds. 

Since degrees of latitude are numbered from the Equa- 
tor as zero, toward each pole, high latitudes are nearer the 
poles and low latitudes near the Equator. All north of the 
Equator is called the northern hemisphere^ and all south 
of it the southern hemisphere. Since latitude is measured 
both north and south of the Equator, there is north lati- 
tude and south latitude^ the United States being in the 
former. Since we know the size of the globe, if we deter- 
mine the latitude and longitude of any given place, we 
can easily tell its exact distance from any other known 
part of the earth. ^ 

The JEarth an Oblate Spheroid. — The diameters of a true 
sphere must be the same in all parts; but that of the 
earth is 7899.1 miles, measured along the axis from pole 
to pole, and 7925.6 miles at the Equator. This shows a 

1 It would be well to spend some time upon this subject, giving the 
students some practice lessons, so that they may fully grasp the meaning 
of latitude and longitude. 



CONDITION OF THE EABTB 7 

slight flattening in the polar regions, and a protuberance, 
or bulging, in the equatorial part, and the sphere is thus 
distorted into an oblate spheroid. In an ordinary study 
of the earth's surface this flattening by about 13^ miles at 
each pole would not be noticed ; but in the movement of 
the earth through space, this deflection from a sphere has 
produced very marked effects. Because of this difference, 
the length of the degree of latitude varies from equatorial 
to polar regions, being less than 68 miles in India, and a 
little more than 69 miles in Sweden. 

General Condition of the Earth. — Speaking broadl}^ there 
are three parts to the earth : (1) the solid earth itself ; 
(2) the partial water envelope ; and (3) the gaseous 
envelope, or atmosphere. 

Air. — The air is in constant movement, performing 
many tasks of importance. We breathe it; it gives life 
to plants and animals ; it diffuses the heat and light which 
reach the earth from the sun; it brings us our winds, 
clouds, and storms ; it furnishes the oxygen by which our 
lamps may burn and our fires glow; it ruffles the ocean 
surface with waves, and drives our ships along ; and in 
many hundred other ways it serves us. Yet the air is 
merely a thin, transparent mass of gas, whose constant 
presence about us is hardly realized (Part II). 

Ocean. — The ocean shuts out from view nearly three- 
fourths of the solid earth, and in places buries it beneath 
a depth of four or five miles of water. Its surface is so 
nearly level (that is to say, it is parallel to the general 
surface of the globe), that we may sail upon it for thou- 
sands of miles without a glimpse of any other irregularity 
than the waves which disturb its surface. 

Like the air, the ocean enwraps the globe, and conforms 



8 FIRST BOOK OF PHYSICAL GFOGBAPHY 

to its general outline, being held in place by the force of 
gravity, by which the earth binds to itself all movable 
objects on its surface. This level water surface, the sea- 
level^ is the plane from which we determine the elevations 
on the land. It is not strictly level, but is slightly dis- 
torted by various causes. 

The Solid Earth. — Some portions of the land are still 
inaccessible, and great areas near each pole have so far 
baffled all the efforts of venturesome explorers; but while 
we have now visited most lands, our knowledge almost 
ceases when we pass below the very surface. Accumu- 
lated on the surface there is generally a soil, and beneath 
this, usually at depths of only a few feet, hard rock of 
various kinds is encountered. Here and there wells and 
mines pierce to the depth of a mile or more, and to this 
depth, at least, the solid rock extends; but we can only 
speculate concerning the conditions below this level. 

In all deep borings and shafts, it is found that the tem- 
perature of the earth increases with the depth ; and while 
there is a considerable variation from place to place, the 
average condition shows an increase of about 1° for every 
50 or 60 feet of descent. If this continues toward the 
centre, as it probably does, the temperature of the earth 
must be very high at the depth of a few score of miles. 
Indeed, it seems that at great depths the temperature 
must be higher than the melting point of rocks at the 
surface. In fact, here and there melted rock comes to the 
air, through cracks reaching down into the earth, and in 
this case volcanoes are formed. 

It was once believed that these facts proved the earth to 
be a great globe of liquid, molten rock, around which was 
a solid rind or crust. But astronomers have shown that 



CONDITION OF THE EABTH 



9 



ATMOSPHERE 



this cannot be ; for in its behavior toward the planets, the 
earth acts like a rigid hody^ and if there is molten mate- 
rial, the outer crust must be 
very thick. Scientists now 
believe that the interior is 
Mghly heated^ but that it is 
kept in a solid condition by 
the great weight of the 
overlying rock.^ We still 
use the term earth^s crust as 
a convenient word to express 
the solid and relatively cold 
outer part of the earth. 

Surface of the Earth : Con- 
tinents and Ocean Basins. — 
When the earth is repre- 
sented by a map or globe, it 
is customary to make the 
surface perfectly smooth ; 
yet we all know that the 
earth's surface is very irreg- 
ular. This is because the 
irregularities with which we 
are familiar are small com- 
pared to the size of the earth. 
While the diameter of the 
sphere is about 7900 miles, 
the greatest irregularity of 
the land above sea-level is 
only about five miles. 




Fig. 4. 
Section to show relative amount of air 
and solid earth, and the supposed 
condition within the earth. 



1 It may be stated that an increase of pressure raises the melting point ; 
and at a depth of several miles in the earth, the pressure of the load of 



10 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



The surface is diversified by a series of grand elevations 
and depressions, the full extent of which is obscured by 
the ocean. The continents are the elevations, the ocean 
beds the depressions. There are two sets of continents, 
the New World, including North and South America, and 
the Old World, including Eurasia, Africa, and Australia. 



NORTHERN HEMISPHERE 

90 



SOUTHERN HEMISPHERE 

90 




Fig. 5. 
The two hemispheres, showing the grouping of continents and oceans. 

On a rather arbitrary basis it is customary to divide these 
land masses into individual continents. The smallest, 
Australia, is quite completely separated from the others, 
though there is a partial connection with Asia by way of 
the East Indies. Europe and Asia cannot be naturally 
separated. Africa is removed from Eurasia only by the 
relatively narrow Mediterranean and Red seas, being con- 
nected at the Isthmus of Suez ; and the two Americas are 
directly connected by the Isthmus of Panama, and partly 
also by the West Indies and the Antilles. 

rock above must be very great — so great, in fact, that melting may be 
impossible. 



CONDITION OF THE EARTH 



11 



Between these groups of continents there are two great 
oceans, the Atlantic and Pacific, while between the Afri- 
can and Australian prolongation of the Old World land- 
group, is another large ocean, the Indian. Around each 
pole there is some land and much water. That around 
the South Pole is called the Antarctic Ocean, and that sur- 
rounding the North Pole the Arctic. Although open to the 
Atlantic, the Arctic is much more enclosed than the Ant- 
arctic, which has no natural boundary line between either 
the Pacific, Indian, or Atlantic. The Arctic may be con- 



^^o^«HIS^^ 



^^55-ti£i:;i!^A^^ 




Fig. 6. 
Land and water hemispheres. 



sidered to be a northern prolongation of the Atlantic, and 
the Antarctic to be parts of the Pacific, Atlantic, and 
Indian. 



Viewing the globe as a whole, we find that the water predominates 
in the southern hemisphere, and the land in the northern. It is also 
noticeable that the water projects, in somewhat triangular tongues, 
from the great nucleus around the South Pole, toward the Xorth 
Pole, and that the continent groups project somewhat triangular 
tongues from the land area of the northern hemisphere toward the 
South Pole. This development of the land in one hemisphere, and 



12 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



the water in the other, makes it possible to divide the earth into two 
hemispheres, in one of which there is little land, while in the other 
the land is distinctly in excess of the water (Fig. 6). These are 
called the land and water hemispheres. 

Not only does the sea cover a greater area than the 
land,^ but the average elevation of the land is much less 
than the average depth of the ocean.^ 




Fig. 7. 

Section across South America, Atlantic Ocean, and Africa, showing 
greater depth of ocean. 

Mountain Irregularities. — The second group of irregu- 
larities are those occurring along relatively narrow lines. 
Upon the continents, ranges and ridges of mountains rise 
above the general level of the land, usually from the crest 
of a high plateau. The most remarkable of these moun- 
tain groups is that facing the Pacific in the two Americas, 
and extending from Alaska to Cape Horn. Occasional 
peaks in these mountains attain an elevation of three or 
four miles above sea-level ; and often, as in the case of the 



1 The area of the earth is not far from 190,700,000 square miles, of 
which about 144,700,000 is water surface and 52,000,000 land, the area of 
the water being about three-fourths of the total. 

2 The average depth of the ocean is computed to be about 12,000 feet, 
while the average elevation of the land above the sea, is only about 2500 
feet, though if the ocean could be removed, the continents would stand 
as great elevations rising, on an average, fully 14,000 feet above the 
ocean beds. In some places of exceptional ocean depth and land height, 
the difference between ocean bottom and mountain peak would amount 
to about 60,000 feet, or over eleven miles, and in a single view there would 
be some cases of elevation amounting to fully eight miles. 



CONDITION OF THE EARTH 13 

Rocky Mountains, the plateau above which they rise is 
fully a mile above the sea. 

At times mountains extend into the ocean, as in the case 
of the Kamtchatka peninsula. By means of these moun- 
tains, great peninsulas and chains of islands, such as the 
Japanese group, partially cut off and enclose arms of the 
sea. Very often, elevations rise entirely in the sea, per- 
haps in mid-ocean, and then, as in the Hawaiian Archi- 
pelago, there are produced chains of oceanic islands. These 
are often the higher peaks of a partly submerged mountain 
range ; and not uncommonly they are volcanic cones, just 
as some of the higher peaks of the mountains on the land 
are volcanoes (Chapter XX). 

Minor Irregularities. — There are many minor irregularities of the 
land, which are mainly the result of the carving of the surface, by the 
weather, rivers, and ocean. By these forces the land is constantly 
being cut into hills and valleys, so that in the course of long periods 
of time, our land surface has become very much worn, dissected, and 
sculptured. Some of the causes for these irregularities are described 
in the chapters on the land (Part IV). 

Movements of the Earth : Rotation. — Every day, over 
most of the earth, the sun rises in the eastern sky, and after 
travelling across the heavens, sets in the west. Between 
sunrise and sunset the earth is bathed in light and heat ; 
at night darkness prevails, and coolness takes the place of 
warmth. Before it was known that the earth was a sphere, 
it was thought that the sun actually rose and set, making 
a daily journey across the heavens ; but for a long time we 
have known that this apjjarent movement of the sun is 
really due to the motion of the earth. Our globe is spin- 
ning about an axis which passes through the poles, as one 
might make an apple rotate by using the stem as an axis. 



14 FIRST BOOK OF PHYSICAL GEOGRAPHY 

This spinning or rotation of the earth is constant and 
quite uniform, and the complete rotation is made in a 
little less than 24 hours (23 hours and 56 minutes), or the 
time between two sunrises. So, as the earth rotates, turn- 
ing toward the east, the sun appears to rise in the east and 
to move across the heavens as the day advances. If we 
could travel across the earth at the Equator, going at the 
rate of about 69 miles in 4 minutes, the position of the 
sun in the heavens would remain the same : starting at 
the sunrise, the sun would remain on the eastern horizon, 
and at the end of the 24 hours we would be at the starting 
place, with sunrise still present; but those who remained 
behind would have experienced the complete changes of 
day and night. 

This is the same as saying that the earth rotates at this 
rate, and that the sun's rays advance over the land in 
this rapid way. But the diameter of a circle of latitude 
decreases from the Equator toward the poles, and there- 
fore the sun's rays creep across the globe at a less and less 
rapid rate as the distance from the Equator increases. It 
takes the earth about 24 hours to make the complete rota- 
tion, whether at the Equator or near the poles ; and hence 
the time between two sunrises is everywhere the same, 
although the distance over which the rays pass from hour 
to hour decreases toward the poles. 

The division of the earth by meridians is based on this 
fact, the distance between two of these lines being that 
which the sun passes in about 4 minutes. This distance 
is greatest at the Equator, and hence the meridians spread 
further apart as the equatorial belt is approached. The 
sun travels from meridian to meridian in 4 minutes ; and 
as there are 24 hours in which to make the journey, this 



CONDITION OF THE EARTH 15 

necessitates 360 meridians on the earth (24 x 60 = 1440 ; 
1440 -^ 4 = 360). For the same reason, if we travel toward 
the west or the east, we find the time to be constantly 
changing, the rate of change being 4 minutes for each 
degree of longitude.^ While the sun has been an hour 
above the horizon with us, 15° west of us it is just rising. 

If a body on the earth at the equator travels over a distance of 
25,000 miles in a day, going at the rate of about 17 miles a minute, 
the question may be asked, Why are not air, water, and all movable 
bodies, left behind and hurled into space ? The answer is that gravity 
draws all things towards the earth and binds them to it. They are 
a part of the earth, moving with it, just as a person becomes a part of 
a train which is whirling along at the rate of a mile a minute. If 
the earth could suddenly stop, all movable bodies would be hurled 
away, just as, when a train suddenly stops, the people are thrown 
forward. 

Revolution : The Sun in the Heavexs. — Though rising 
and setting every da}^, the sun each morning rises in a dif- 
ferent place from that of the preceding day. This is scarcely 
noticeable in two succeeding mornings, but from month 
to month is distinctly seen. The sun slowly changes 
its path through the heavens, now being low, again high 
at midday; and as this path changes, our seasons vary. 
In about 365 days (365.24 days) the cycle of seasonal 

1 In this country, in order to avoid the confusion resulting with every 
town having its own true or solar time, artificial boundaries have been 
drawn parallel to the meridians, so that at distances of 15° the time 
changes one hour, while all places between two such meridians have the 
same standard time. There are several such belts, and now, when we 
travel across the country, we are obliged to set our watches when we 
come to the boundaries, setting them back on the journey west and ahead 
when going eastward. In this country there are five divisions of Standard 
time as follows: Intercolonial (52|"-67i°), Eastern (67i°-82i°), Central 
(82^°-97^°), Mountain (97i°-112L°), and Pacific (112i°-i27^°). < 



16 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



changes — the year^ we call it — has been passed through ; 
and then we again go over the same cycle. 

If we could spend a year at the equator and others at 
various points between this and the poles, we should find 
an entire difference in the seasons of the several places. 
In each place the sun would have a new series of move- 




FiG. 8. 
To illustrate day and night at equinox, when the sun's rays reach both poles. 

ments, but in a single locality^ the cycle would be the 
same year after year. 

At the equator the sun would rise nearly in the east, 
pass almost directly overhead at noon, and set in the west. 
During the season which corresponds with our winter, 
the midday sun would be somewhat south of the zenith, 
and during the season corresponding to our summer, it 
would be an equal distance north of the vault. Passing 
23^-° northward, we should find the sun to be always south 



CONDITION OF THE EARTH 17 

of the zenith, excepting in midsummer, when it would ex- 
actly reach the zenith at midday ; in midwinter it would 
be furthest south. Passing north of this, the sun would 
always be found in the southern half of the sky. At 
midday, in winter, it would be very low, while at the same 
time, the days would be short and the nights long. 

These conditions continue to increase until within 23|^ 
of the pole, where the midsummer sun rises fairly high 
in the heavens, but in midwinter just reaches the southern 
horizon. Beyond this the sun does not generally have a 
daily rising and setting, but remains above the horizon for 
weeks, and further north for months at a time. Then it 
passes below the horizon to stay during the long, cold 
winter night.^ What is said of the northern hemisphere 
is equally true for the southern, if we change south to 
north, and north to south. Our winter is the southern 
summer, and vice versa. 

Cause of Seasons. — These peculiarities are the result 
of a second movement of the earth, its revolution around the 
sun. Although the sun is on the average about 92,800,000 
miles distant, the tie of gravitation^ which extends through- 
out the solar universe, keeps the sun and earth together, 
while the latter revolves around the former, whirling 
through space at the rate of 1000 miles a minute, and 
making the complete journey in a year, and year after year 
going over approximately the same path. If it were not 
for the revolution, and the earth were merely a rotating 

1 During the summer the sun circles near the horizon, dipping toward 
it at night, when it is near the north (Fig. 71), and rising higher at mid- 
day, when it has circled into the southern quadrant. Between the winter 
night and summer day there are short seasons when the sun does actually 
rise and set. Exactly at the pole the sun is above the horizon half the 
year, and below it the other half. 
c 



18 FIRST BOOK OF PHYSICAL GEOGRAPHY 

sphere fixed in space, we could have no seasons, but each 
day would be like the preceding. The same would be 
true if the earth revolved about the sun with its axis ver- 
tical to the plane of revolution. Then the sun's rays 
would reach the equator over the zenith at noon of every 
day in the year, and north of the equator, at any given 



i^^^^^^^^^^BK^^^^^ V A'^mpj^i 


l^^l 




^^^^BHP^H^^^^^^^^^^^H 




'v«^vt,X^„^^i^^^^^^^^ll 








^'^^^^^H 


W 1] 


'^9 


^^^B^^^hai^ r 4 


\4|C£\^^^^^^H 



Fig. 9. 

To illustrate conditions in northern summer when the whole Arctic is 
bathed in sunlight. 

place, every day would find the sun in the same part of 
the heavens, and the sunrise and sunset would always be 
at the same place. As the poles were approached, the sun 
would be lower and lower in the heavens, until at the 
exact pole, it would be seen making a complete circuit of 
the horizon. This is exactly what happens twice each 
year, at the time of the vernal and autumnal equinoxes 
(the spring and autumn, March 21 and September 22 



Vi 



CONDITION OF THE EARTH 



19 



respectively), when the day and night are equal in length, 
each being 12 hours (Fig. 8). 

In reality, the earth revolves with its axis inclined at an 
angle of about 231° (exactly 23° 27' 21'') to the plane of 
revolution, and it 
is because of this 
that we have the 
seasons. Imagine 
the earth fixed in 
space and rotating 
about an axis in- 
cHned 23J° to the 
plane of revolution 
of the earth about 
the sun. Suppose 
that the North Pole 
is inclined away 
from this plane, 
and the South Pole 
toward it (Fig. 10). 
Then the sun will 
be vertical at Lat. 




Fig. 10. 

Condition during northern winter when the sun's 

rays just reach the Arctic circle. 



231° south of the Equator, or over the Tropic of Qa'pricorn. 
The whole of the south polar region will be bathed in 
light, giving perpetual day in that part of the earth. All 
the southern hemisphere would be light. In the northern 
hemisphere the sun will everywhere be in the southern 
heavens, and at a distance of 231° from the North Pole, 
the solar rays will cease to light the earth, while beyond 
this line, which forms the Arctic circle^ perpetual night 
will prevail. 

If on the other hand, the axis is turned with the North 



20 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Pole toward the sun (Fig. 9), the reverse will be true, and 
the sun's rays will always be vertical at noon over the 
northern tropic, Cancer, which is 23^° north of the Equator. 
Beyond the Antarctic circle, or a distance of 23 1° from the 
South Pole, a condition of perpetual night is present. If 
this position were maintained, the one hemisphere would 
have per]3etual winter, the other perpetual summer; and 
the temperature would decrease from that tropic over 
which the sun was vertical, toward each pole. 

Since the axis is inclined, and is j^ear by year pointing 
toward nearly the same place in the heavens ^ (the north 
pole pointing approximately toward the North Star), the 
revolution of the earth about the sun turns the North 
Pole now toward and now away from the sun (Fig. 11), 
and so the two hemispheres enjoy alternation of seasons, 
and are thus treated alike.^ This is the same as saying, 
that when the South Pole is turned from the sun, and 
when winter prevails in the southern hemisphere, we in 
the northern hemisphere have long summer days with the 
sun high in the heavens at noonday. This then gradually 
changes to autumn, when the days become equal in length, 
and our sun is less high in the heavens. At this time, the 
rays of the sun cover the entire earth, which is the condi- 

iln the course of long periods of time this does change, but this is a 
question of astronomy which does not bear especially upon the present 
subject. 

2 It is very commonly the case that the pupil merely memorizes these 
facts without really grasping the fundamental principles ; but the teacher 
should see to it that each student really understands these points. This 
can be easily done if the teacher will make intelligent use of a globe, 
or of any spherical body, showing the way in which the axis maintains 
its position while the earth moves, and the hemispheres face toward and 
away from the sun in the different seasons. 



CONDITION OF THE EARTH 



^1 



tion that would exist if the earth's axis were at right 
angles to the plane of revolution. 

Gradually the sun takes a lower position in the heavens, 
the day shortens, and midwinter is reached, while at the 
same time summer prevails south of the Equator. Then 
begins the return of warmth with the spring and lengthen- 




"VemaLEipmaJC 

Fig. 11. 

To show change in seasons as the earth revolves about the sun (ellipse exag- 
gerated and relative size and distance not shown) . 

ing days. Soon the yearly cycle is over, because the revo- 
lution is complete, and the sun has come back nearly to 
the place where it started the year before. As soon as 
one revolution is finished a new one is begun, and so year 
after year the earth pursues its path about the sun, and 
year by year we find the same alternation of seasons. So 
distinct is this cycle, that astronomers are able to predict 
just what the position of the sun, and the length of the day, 
will be a hundred years from now. 



CHAPTER II 

THE UNIVERSE 

The Solar System : The Sun. — The earth is but one of 
a great family, all having certain resemblances, and all 
bound together by the common bond of gravitation. They 
pass through space ^ in company, yet each performs certain 
duties and movements of its own. The central body is the 




Fig. 12. 

To show that of all the light and heat sent from the sun in all directions, the 
earth receives hut a very little. 

^ Space is the great unknown expanse which surrounds the earth, and 
so far as we know, extends without limit in all directions. We are unable 
to conceive of anything without an end, and yet we are unable to con- 
ceive of an end to space : space baffles our most acute perception. It is 
believed to be empty of all substances with which we are acquainted ; yet 
since light and heat pass through it, it is thought to be pervaded by a 
mysterious ether^ which allows waves of light and heat to pass from the 
sun to the earth. Its temperature is believed to be very low, perhaps 200 
or 300 degrees below zero. 



THE UNIVERSE. 23 

sun, a hot glowing mass, apparently composed of the same 
elements as the earth itself, but so highly heated that both 
heat and light are emitted in all directions into space 
(Fig. 12). A small part of this is intercepted by the 
earth as it moves around the sun, and this form of energy 
performs work of immense importance. Like the earth 
itself, the sun is a great spherical body, its diameter being 
about 860,000 miles, or more than 100 times that of the 
earth. If the centre of the sun were within the earth, 
its body would not only cover all the space between 
us and the moon, but it would extend two-thirds as far 
beyond. 

The Planets.^ — While the earth revolves around the 
central sun, and receives its light and heat from this 
source, it is not alone in this, for there are other great 
spheres which also revolve about the sun in orbits which 
bear a general resemblance to that pursued by the earth. 
These are colled planets, and there are eight of these, which, 
named in the order of their distance from the sun, are Mer- 
cury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and 
Neptune. They are all somewhat flattened spheres (oblate 
spheroids), revolving in nearly circular elliptical paths 
about the sun, and those that are well enough known 
have been found to have a rotation about an axis. In the 
heavens they shine as stars ; but their light is reflected 
from the sun. Some, at least, have an atmosphere, but at 
present our knowledge of most of the planets is very slightc 

Jupiter, the largest of these (86,000 miles in diameter), 

1 Besides the planets there are smaller spheres, called asteroids, in the 
space between Mars and Jupiter. The largest is about 520 miles in diam- 
eter, and the smallest less than 40 miles. There are also comets and shoot- 
ing stars moving in the space occupied by the solar system. 



24 FIRST BOOK OF PHYSICAL GEOGRAPHY 

has a mass greater than all the others combined ; but it has 
only one-tenth the diameter of the sun. On the other ex- 
treme, Mercury, the planet nearest the sun, has a diameter 
of about 2992 miles, being only a little less than one-half 
as large as the earth. The other planets range in size 
between these two extremes. While the distance of 
the earth from the sun averages about 92,800,000 miles, 




Fig. 13. 

Diagram to show relative distance of the planets from the sun, and also 
their relative sizes. 



Mercury is only 35,750,000 miles distant, Jupiter about 
480,000,000, and Neptune, the most distant of the planets, 
is 2,775,000,000 miles away. In travelling through these 
immense distances, in their journey about the sun, the 
earth occupies 365 days. Mercury 88 days, Jupiter 12 of 
our years, and Neptune about 165 years. 

Satellites. — While each of the planets is revolving 
about the sun, all but two of them, Mercury and Venus, 
have smaller bodies revolving around them. These Satel- 
lites vary in number, Saturn having eight. 

The earth's satellite, the moon, is a cold sphere with a 
diameter of about 2160 miles, and an average distance from 
the earth of about 240,000 miles. In company with the 
earth it moves about the sun, and as it goes, makes the 
journey around the earth every 29J days. When it shines, 
it does so by reflected sunlight. 



THE UNIVERSE 



25 



Being so near the earth, the moon has been carefully 
studied by the aid of powerful telescopes, and we know 
more about its surface than we do about any other body in 
space. Only one side is turned towards us, and we never 
see the opposite face. On that side which we see there is 
neither water nor atmosphere : its surface is very rough, 




Fig. 14. 
Craters and ridges on the moon. 



and many of the irregularities are great crater-like pits, re- 
sembling immense volcanic craters (Fig. 14). It is thought 
by many that these are craters of ancient volcanoes which 
are now extinct. Thousands of them, great and small, 
pit the surface of the moon. 

The Universe. — While the earth seems so large to us, 
and while most of us see but a tiny part of it, and know of 
the rest only from the description of others, it is, relatively, 
but a tiny speck of dust in the great universe upon which 
we gaze every starry night. When we look upon a twink- 
ling star, we see a sun so distant that the very light which 



26 FIRST BOOK OF PHYSICAL GEOGRAPHY 

meets our eye may have left the star hundreds of years 
ago ; and perchance the star that we see, no longer exists. 

In the Milky Way also, which to the unaided eye looks 
like the gleam of the sun's rays reflected from a thin cloud 
in the upper air, the telescope finds myriads of stars, one 
beyond the other ; and beyond these are still others which 
even the telescope cannot distinguish. In this belt of 
abundant star-dust, the limit of suns cannot be told ; and 
yet all of these bodies are so fixed in space that year by 
year their position appears to be unchanged. 

What is this space, the nature of which no man can 
even guess, and the limits of which no man has yet been 
able to find? Can those who believe that they know the 
origin of the universe, and who think that they can re- 
duce it to a system of natural laws, throw any light upon 
this? If so, they have yet to announce an explanation 
which satisfies the average mind. There is something in 
this wonderful system that may well cause the human 
mind to recognize its own smallness and insignificance. 

In the heavens there are large clusters of stars, which 
to the eye are unknown, but which the telescope reveals ; 
and these may be other stellar systems, like that which we 
view when we look into the star-lit vault of the heavens. 
Is our stellar system, of which the great solar system is 
but a small part, itself a small portion of a great universe 
of mani/ stellar systems ? Here again no answer can be 
given. Are the tiny stars each a mother sun, with a fam- 
ily of planets ? and if so, do these planets resemble ours, 
and are they inhabited by life ? Again we cannot even 
guess : but why may not this be so ; for is it probable that 
in all this great and apparently endless universe, our tiny 
earth is the only favored spot ? 



I 



THE UNIV^BSE 27 

The power of the human mind is indeed restricted, for 
we learn by experience. The infant reaches out to grasp 
objects that are far beyond its reach ; the child of two or 
three will try to make an object pass through a space 
smaller than itself, and will learn better only by repeated 
experiments ; the boy of ten or fifteen, who has known 
only his own town or country, can have only a slight con- 
ception of the size of the earth ; and the man, accustomed 
to measure by his experiences on the earth, can have no 
proper conception of the distance of the sun, and cannot 
even dream of the meaning of a billion miles. 

The best that one can do, in lieu of our inability to really conceive 
this, is to become impressed with the immensity of these distances of 
space by some comparison with things of ordinary experience. An 
express train in most cases goes no faster than 60 miles an hour, 
and as it passes us, it comes and goes with a rush that is almost 
startling. Let us suppose that we could start on a journey from the 
sun to Neptune, passing the earth, and the moon, and travelling con- 
tinuously at the rate of 60 miles an hour. At this rate of speed, it 
would take 17 days to travel around the earth at the Equator. Start- 
ing at the sun, a little more than 176 years would be occupied in reach- 
ing the earth. A little more than 166 days would take us to the moon, 
and the journey from the sun to the planet Neptune would require 
5280 years. To reach the nearest star would take several hundred times 
as many years. That is to say, if one had started from the sun at the 
beginning of the Christian era, he would still be journeying, and 
would be only part way between Saturn and Uranus ; yet during this 
period of time a great part of the recorded history of the human race 
has taken place. 

The Nebular Hypothesis : Symmetry of the Solar System. 
— Reviewing the conditions shown by astronomers to 
exist in the solar system, it is found that the regular 
members are all spherical bodies, and those that are near 



28 FIRST BOOK OF PHYSICAL GEOGRAPHY 

enough to have been studied carefully, show a flattening 
in the polar regions. All that are well enough known, 
show a rotation about an axis passing through these 
flattened parts of the sphere; and all of them revolve 
about the sun in the same direction. The axes of 
rotation are all inclined to the plane of revolution. The 
satellites show similar uniformity ; and in addition they 
are revolving around their parent planets. These move- 
ments are all so regular that astronomers can predict in 
advance exactly what they will be. The paths pursued 
by these bodies are all nearly circular ellipses, at one of 
the foci of which is situated the central body, the sun, 
around which the revolution is made. There is therefore 
a wonderful symmetry of form and movement of the 
spheres ; all obey the laws of gravitation, by which they 
are all bound together in this regular, well-established 
system of movement. 

There is harmony also in other respects. Astronomy 
tells us something of the composition of the sun, and in 
this are found some of the very elements which compose 
the earth. There appears to be a progressive decrease in 
heat as the size of the sphere diminishes. The sun, the 
largest, is intensely hot ; Jupiter, next in size, is apparently 
warm, but is not luminous at the surface ; the earth is 
cold at the surface, and hot within ; the moon appears to 
be cold throughout its entire mass. Again, from Mercury, 
the planet nearest the sun, to Neptune, the most remote, 
there is an almost uniform decrease in density of the ma- 
terials composing the planets. 

The Explanation. — It has seemed to men that these 
conditions called for a uniformity of origin ; and before all 
of these facts were known, philosophers and astronomers 



THE UNIVERSE 29 

had proposed the brilliant explanation for the solar system 
which we know as the Nebular Hypothesis, This is still 
held by astronomers, and many new facts have been 
brought to its support. While it cannot be said to be 
more than a theory, it has the advantage of explaining 
nearly all the facts, while there is little to oppose it. It 
is now more firmly grounded than ever before. 

Briefly, the jSTebular Hypothesis is this : In the begin- 
ning, the solar system was a mass of glowing gas, slowly 
revolving in the direction which the planets now pursue 

A B 



Z) c 



. a 
7 





Fig. 15. 

Diagram to illustrate Nebular Hypothesis. A mass of heated gas {A) more 
dense at centre (c) is rotating in the direction of the arrows ; in 5 a ring 
ah is thrown off, more dense at a than elsewhere; in C this ring has gath- 
ered {K) around a centre (a in B) and has itself thrown off a ring {i), 
while another ring (e) has come off from the central mass ; in Z) a planet 
(P) and Satellite (S) have formed, and these are revolving in the direction 
of the arrows. The ring (e in C) have also gathered into spheres 
(3/andiV). 

in their movement around the sun. The mass was cool- 
ing by the radiation of the heat into space, just as the sun 
and earth are now still losing heat. There was a certain 
loss of bulk from contraction, and in the course of time 
this caused the parent mass to throw off rings, some- 
thing like those which rise from an engine as it is start- 
ing from a railway station. These continued to slowly 
revolve in the original direction, and gravity gradually 
drew the mass of each ring together into a spherical body, 
about some portion which was originally more dense 



30 FIRST BOOK OF PHYSICAL GEOGRAPHY 

than the rest. The sphere of gas continued to revolve 
about the parent nebula, and to rotate as it went. Some 
of these spheres have themselves thrown off rings, which 
upon passing through the same history began to form 
spheres, which revolved around their parents. 

As the heat became less intense, in the course of time 
these began to solidify, the smallest, and those that were 
first thrown off, being the first to reach the solid stage. 
Therefore the sun, which is the largest and most central 
body, is the hottest, while so small a body as the moon, 
and so distant a planet as Neptune, are the coldest. 

Facts accounted for. — This hj^othesis accounts for the uniformity 
of rotation and of revolution : it explains the spherical form, because 
gravity, acting upon a gaseous body will necessarily produce a sphere.^ 
It explains the flattening at the poles, because, by the centrifugal 
force, a rotating sphere of gas, or of liquid, will bulge at the Equator, 
where the rotation is most rapid. It accounts also for the heat of the 
sun and the earth's interior. It also explains the decrease in density 
from the inner to the outer members of the system ; for the first rings 
thrown off would be composed of the less dense outer portions of the 
nebula (just as the air and water of the earth are outside of the 
denser crust) ; and finally, it tells why the sun and the earth contain 
the same elements. At the same time it must be understood that 
this satisfactory explanation depends upon some very distinct assump- 
tions, and it presupposes that there was a nebulous mass of hot gas, 
revolving and under the influence of gravitation. 

If this explanation is correct, the earth is descended from a much 
hotter body, and has now reached a stage in cooling when only the 
interior is hot; and it will continue to lose heat until the condition of 
the moon is reached. In time, in the course of indefinite ages, the 
sun also will lose its heat, and our globe will be cut off from the supply 
of heat and light energy which are of such vital importance to all life. 

1 This may be illustrated by a globule of oil in water, and the flatten- 
ing may be shown by revolving such a sphere. 



THE UNIVEUSE 31 

Other ISfehulce. — Far away in space, the telescope has 
revealed masses of glowing gas like that which the Neb- 
ular Hypothesis conceives; and some of them show the 
condensing rings and spheres, like those supposed to have 
existed when the solar nebula was forming. From this it 
seems possible, that in the far-away confines of space, other 




Fig. 16. 

The Andromeda nebula showing rings and denser parts in a 
nebulous mass. 

worlds are even now in process of formation. Whether 
true or not, it is a beautiful hypothesis. It is an attempt 
of the human mind to explain the most profound mystery 
of nature, — to account for the wonderful law and sym- 
metry that everywhere prevails; but the mind, although 
always impelled to attempt explanation, is liable to error, 
and is very limited in its power of conception. 



I 



PART IL — THE ATMOSPHERE 

CHAPTER III 

GENERAL FEATURES OF THE AIR 

Importance of the Air. — An invisible ocean of elastic 
gas surrounds the earth with its life-giving substance. 
It fans the surface with breezes and disturbs it with 
violent winds. It carries invisible vapor from water to 
land, where it falls as rain. It spreads light and warmth 
over the globe, and in many ways its presence works to 
the advantage of the varied life that overspreads the earth. 
Although invisible, the air has substance, and when it is 
in motion we feel the breeze or wind. We breathe it, 
and it gives to us materials that are necessary for our 
existence. 

Place an animal in a small enclosed space, and it soon exhausts the 
air, and although a gas still remains, it is different from the original. 
The breathing has caused a chemical change, and unless new air is 
furnished the animal dies. A candle placed in a similar position will 
soon cease to burn, and it is then found that the gases of the air have 
been changed by the burning of the candle. 

If an animal should be placed under a closed cylinder on an air 
pump, and the air be exhausted, death would soon result, for there 
would be no air to breathe and perform the work which the body con- 
stantly demands of it. For a similar reason a person suffocates and 
drowns when kept for a few minutes under water, which excludes the 
air from the lungs. 

32 



GENERAL FEATURES OF THE AIR 33 

Composition : Oxygen and Nitrogen. — Careful study has 
shown that the air is made chiefly of two gaseous elements, 
nitrogen and oxygen^ about 21% of the latter to 79% of the 
former. 1 

In the air, nitrogen (and also argon) is a very inert ele- 
ment, which acts as an adulterant to the active oxygen, 
in a manner similar to the adulteration or weakening of a 
solution of salt when water is added to it. It is oxygen 
that is doing the work in the bodies of animals, and caus- 
ing many changes on the earth. Nevertheless nitrogen 
is a very important part of the atmosphere ; for if it were 
absent, the bulk of the air would be very much less, and 
the work of the oxygen more rapid. An animal cannot 
live in pure oxygen, for this works more rapidly on the 
tissues than does the adulterated oxygen of the air.^ 

Carbonic Acid G-as. — In a burning candle or lamp, the 
oxygen of the air is producing a chemical change in the 
burning substance. If we should exhaust the oxygen, 
the light would go out. If more oxygen were added, the 
light would burn much more brilliantly. This combustion 
or oxidation is somewhat like that which takes place when 
a man breathes the life-giving oxygen into his lungs, for 
then also the oxygen gas combines with other substances. 

1 In 1894 a new gaseous substance, called argon, was discovered in 
the atmosphere, of which it forms a considerable proportion. It may 
appear strange that an element which we have always been breathing 
should so long have escaped detection ; but this new element resembles 
nitrogen so closely that the two have been co^afused. At present it is 
impossible to tell much about the new gas. 

2 It is something like the difference between a fire with the draft closed 
and one with an open draft, through which more oxygen is furnished, thus 
causing more rapid burning. The teacher can easily show this by an 
experiment, making and collecting oxygen in a receiver in which a candle 
is burning. 

i> 



34 FIRST BOOK OF PHYSICAL GEOGBAPHT 

In a lamp, and in the lungs, oxygen combines with 
carbon, producing the gas which is known as carbonic acid 
gas (carbon dioxide). This is why a lighted candle in a 
small jar soon ceases to burn ; for after awhile all of the 
oxygen is consumed by combining with the carbon of the 
candle, and its place is taken by the newly made carbonic 
acid gas. For the same reason an animal cannot live long 
in a closed space a little larger than itself. 

Growing plants perform the reverse work of converting 
carbonic acid gas back to oxygen. They need carbon, 
and they take it, furnishing in return pure oxygen. 
Hence in a measure they act as purifiers of ihe atmos- 
phere, destroying some of the carbonic acid gas made by 
animals, and replacing it by oxygen. 

But carbonic acid gas forms an appreciable part of the air 
(about .03% of the whole), and it is everywhere present. 
There are several sources from which it is known to come. 
When breathing, every animal is furnishing some, and 
everything that burns supplies this gas to the air. Every 
animal and plant that is dead and decaying is giving out 
this gas, and a supply is also obtained from the earth 
itself. Much carbon is locked up in the earth, as for 
instance, where plants have not decayed but have been 
changed to mineral coal. We burn this in our stoves, 
and one of the products is carbonic acid gas. It is also 
constantly escaping from many springs, and probably 
also from the soil. Also when a volcano breaks forth 
in eruption, large quantities of this gas escape. Because 
of the large amount of fuel burned there, more carbonic 
acid gas exists in the air near cities than in the open 
country. 

This gas serves well to illustrate how beautifully every- 



GENERAL FEATURES OF THE AIB 35 

thing is adjusted to the existence and development of life 
on the earth. Here, for instance, is a gas, forming only 
.03% of the entire atmosphere, which if decidedly in- 
creased, or slightly diminished, would be fatal to all 
animal life on the land. If very much increased, it 
would be directly fatal ; if diminished, the plant life that 
depends upon it for existence would perish; and as the 
animals of the land cannot take their food directly from 
the earth, but obtain it entirely by means of plants, the 
destruction of these would necessitate the death of all 
animals excepting those that could draw entirely upon 
the ocean for their subsistence. But for untold ages this 
harmony of nature's balance has been maintained, and the 
earth has been clothed with vegetation and occupied by 
myriads of animals, great and small. 

Water Vapor. — While there are minute quantities of 
many other gases in the air, there is but one other really 
important gaseous constituent. When wet clothes are 
placed upon the line to dry, little by little the water dis- 
appears, until finally none is left. Also after a rain, the 
pools of water in the road slowly disappear, the mud dries 
up, and the water is gone. In both cases it has evaporated, 
and has changed its form from the visible liquid to the 
invisible gas which we call water vapor (Chapter IX). 

This process of evaporation is somewhat like that which 
is producing steam. The kettle on the stove boils, steam 
issues from the neck, and in time the kettle becomes dry, 
the water having changed to vapor, which is still present 
in the air of the room, though no longer to be seen. That 
it is present may be shown on a frosty day ; for then when 
the vapor-laden air of the room encounters the cold win- 
dow, some of the vapor is condensed back to water, forming 



36 FIRST BOOK OF PHYSICAL GEOGRAPHY 

drops on the glass. ^ If the day is very cold, it solidifies 
into fantastic frost crystals, the solid, icy form which water 
takes when the temperature has descended below the 
freezing point. Even the breath may furnish enough 
vapor to cause this, and on cold nights our chamber 
windows are covered with frost. 

The housewife knows that some days are better drying 
days than others. When the warm sun shines upon the 
clothes, they generally dry quickly, for evaporation takes 
place more rapidly in warm than in cold air. But heat is 
only one of the aids to evaporation, and this is illustrated 
by the fact that some of the hot, muggy days of summer 
are not such good drying days as the cold, windy spells 
of winter. This is because the air cannot contain more 
than a certain quantity of vapor, and on the muggy 
summer days the air is nearly saturated^ while it is rela- 
tively dry during the cold, windy days of winter. Because 
it moves the air, the wind also favors evaporation, and 
thus does not allow it to remain near the damp object 
long enough to become saturated. 

The amount of water vapor that the air can contain 
depends upon the temperature,^ warm air being able to 

1 The teacher may illustrate this by bringing a pitcher of ice water into 
a warm room. 

2 The comparison may be made (though it is only partly analogous) to 
a salt solution. If several spoonfuls of salt are placed in a dish of cold 
water, all of it may not dissolve. Heating this water, more salt is taken 
into solution, and then if the salt water is allowed to cool, some of the dis- 
solved salt will be precipitated in the form of crystals. So it is with the 
air ; cold air can contain little vapor, warm air will hold more ; and if 
this is then cooled, some of the vapor may be forced to assume the liquid 
or solid forms of rain, fog, dew, or frost. 

When saturated, at ordinary pressure, a room 10 feet high and 20 feet 
square contains 346 pounds of air, if the temperature is 0°. In this, if 



GENERAL FEATURES OF THE AIR 37 

carry more than cold. Nevertheless, air will carry some 
vapor even when its temperature is below the freezing 
point. This is illustrated on a cold winter day, when the 
clothes freeze upon the line but still continue to dry. 
The rate of evaporation depends partly upon the dryness 
of the air; for just as a saturated solution of salt cannot 
be made stronger without increasing the temperature, so 
air, having as much vapor as it can hold, will take no more, 
while dry air greedily absorbs it.^ Hence dry air evapo- 
rates water more rapidly than nearly saturated or humid 
air. If of high temperature, more can be evaporated than 
at lower temperatures, and if moving, it takes vapor more 
readily than if quiet. 

Although present in very small quantities, forming 
only a small proportion of the entire atmosphere, water 
vapor is one of the most important constituents of the air. 
Even in the driest parts of the land it is always present, 
although then in very small quantities. The conversion 
of water vapor back to water or snow is constantly in 
progress. Every cloud, every fog particle, every glisten- 
ing drop of dew, and every drop of rain or snow crystal, is 
a witness of this remarkable transformation; and as it 



saturated, there is an amount of vapor which, transformed to water, would 
weigh one-third of a pound. If the temperature is increased to 60°, and 
the air still saturated, its weight is 301 pounds, and the vapor when con- 
densed would weigh 3i- pounds. If the temperature is raised to 80°, the 
air weighs 291 pounds, and the vapor if condensed, 6i pounds. A pound 
of water equals about one pint. 

1 For purposes of graphic description it is convenient to make this com- 
parison ; yet physicists know that evaporation would occur if there were 
no air, for it depends not upon air, but upon the water ; but the air is 
importantin evaporation, because it bears the vapor away and also warms 
the water by its presence. 



38 FinST BOOK OF PHYSICAL GEOGRAPHY 

silently and almost mysteriously proceeds in the change, 
a work of vital importance is performed. 

It sprinkles the land Avith showers, causing countless 
myriads of plants to burst forth into leaf, flower, and fruit. 
It transforms the salt water of the sea to fresh drops of rain ; 
and this, in our rivers, lakes, and springs, furnishes the 
water supply upon which we are so dependent. Without 
this ingredient of the great atmospheric ocean, the earth 
would be a desert sphere whirling aimlessly through space. 

Bust Particles. — Even more minute in quantity than 
either of the gaseous elements of the air is the solid con- 
stituent. The solid particles that float about in the air 
are commonly known as dust; and when a beam of light 
enters a room, the larger dust particles are seen dancing 
to and fro. In the term dust are included many differ- 
ent particles which are so light that they may float in the 
air. Some are visible to the eye ; others, and the majority, 
are microscopic in size. 

When wood is burned, carbon combines with oxygen to 
form carbonic acid gas ; but there are some solid particles 
which do not become transformed to gas. Portions of 
this are left behind as ash, while some rise into the air 
and float away, as we may see by watching the smoke 
rising from a chimney.^ In large cities so much smoke 
is sent into the air, that a dull cloud hovers over them, 
and the sun shines less intensely than in the open coun- 
try. Dust is also blown into the air from the ground, and 
there are many microscopic microbes, and quantities of tiny 
solid substances of various kinds. 

1 That solid particles are rising from even the blaze of a candle may be 
proved by holding a piece of glass over the flame and watching it become 
covered with soot. 



GENERAL FEATURES OF THE AIR 39 

So much dust comes from these various sources, that if 
allowed to accumulate, the air would in time become so 
impure that the sun's rays would be obscured, perhaps 
even more than they are in the large cities, on days when 
the air is quiet ; but the dust is constantly being removed, 
some by slowly settling to the earth, some by the action 
of rain, which as it descends through the air, catches and 
carries it to the ground. So the rain purifies and fresh- 
ens the atmosphere ; and if a raindrop is examined under 
a powerful microscope, it is found to contain many minute 
solid bits. 

The amount of dust varies greatly; at times the air is 
clear and free from these impurities, and then the sun 
shines brightly and the sky has a beautiful blue tint. 
Again, particularly during drouths, when forest fires are 
common, and rains have not come to remove the solids, 
the air is hazy and the sky and sun dull, sometimes almost 
obscured. At such a time the raindrops of a slight 
shower contain enough dust to discolor white paper. 
Over some cities where soft coal is burned, the dust of 
the air settles in sufficient quantities to discolor white 
objects. 

Although near cities there is more dust in the air than elsewhere, 
there are times in desert regions when sand particles, even of consid- 
erable size, are whirled into the air by the wind, and kept there by its 
motion, producing sand storms which shut out from view even the 
objects close at hand. These days are exceptional, and the sand soon 
settles when the air again becomes quiet. A violent volcanic erup- 
tion also causes dust to spread high into the air, and this at times 
travels for thousands of miles before settling to the earth, while near 
the volcano the darkness of night may be produced at midday. Over 
the ocean there is less dust than over the land, and high in the atmos- 
phere, and on mountain peaks, the air is purer than on the lowlands. 



40 FIBST BOOK OF PHYSICAL GFOGBAPHT 

Dust particles are of much importance in the action of 
the air. The microbes which are present, spread disease. 
The solid particles appear to serve as nuclei around which 
vapor condenses to form fog particles and rain ; and their 
presence in the atmosphere is responsible for many of the 
effects of sky and cloud color with which we are familiar. 

Height of the Atmosphere. — It is not to be supposed 
that the upper limits of the air are sharply defined, like 




Fig. 17. 



To illustrate the decrease in density of the air from sea level to 
the higher regions. 

the surface of the liquid ocean. So far as we know, the 
atmosphere becomes less and less dense as the distance 
from the ground increases, and probably gradually fades 
away, until there is an almost indefinite boundary separat- 
ing it from the great void of space. No one has ever been 
near this limit, and so we can only conjecture as to its 
nature; but that there is some such boundary between 
empty space and terrestrial atmosphere, is proved by the 
behavior of meteors and shooting stars. 

These wanderers in space, though sometimes large, are 



GENERAL FEATURES OF THE AIR 41 

usually tiny particles, which, like the earth, are moving 
in an orbit around the sun, travelling also with terrific 
velocities. When in space they are cold, and to us invis- 
ible; but when they cross the path of the earth, and 
encounter the gases of the atmosphere, the friction causes 
heat, just as heat is generated when a knife is held upon 
a dry grindstone that is revolving. The meteors then begin 
to glow, and in most cases to finally burn up and dis- 
appear. They flash out suddenly as brilliant beams of 
light, and leave behind a track of fire, which itself quickly 
disappears. This furnishes proof that the meteors come 
from a place where nothing impedes their passage, to one 
Avhere they encounter resistance, which, though only that 
caused by an invisible gas, nevertheless serves to destroy 
all but the largest, which sometimes fall to the earth. 

In balloons, and on mountain tops, men have ascended 
to a height of five or six miles above the level of the sea. 
Air is still found, but it is so light and rarefied that 
breathing is difficult, and such ascents are sometimes 
dangerous even to life. The air has definite weight, 
which can be measured (Chapter YII) ; and from meas- 
urements at various heights, we know that more than one- 
half of its whole bulk rests within four miles of the earth's 
surface, and fully two-thirds within the lower six miles 
(Fig. 17). But while so large a percentage of its total 
bulk is near the earth, the atmosphere is not limited to so 
shallow a depth. As the elevation becomes greater, the 
molecules of gas become further and further apart, and 
the air less dense. The measurements of the height at 
which shooting stars begin to glow, seem to prove that 
there is some air at an elevation of fully 500 miles from 
the surface (Fig. 4). 



42 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Changes in the Air. — Already it has been hinted that 
the atmosphere is elastic and mobile, and that there are 
many changes in progress. The heat from the sun warms 
the earth and air, heating some parts more than others, 
warming by day, and allowing cooling at night, causing 
intense warmth in some seasons and allowing the opposite 
season to be cool or cold. So the temperature varies from 
one point to another, and from day to day, as well as from 
season to season. Since the air is elastic and easily dis- 
turbed, these differences in warmth cause movements; 
and so the air is in constant motion, now violent, now 
moderate (Chapter VII). 

Water evaporates from the land and the seas, and the 
changes in temperature and movement of the air cause 
dry winds to-day, and possibly damp winds to-morrow. 
The vapor condenses, forming dew, or possibly clouds; 
and even storms may develop, moving across the land and 
causing rain to fall. So the air is ever variable, and no 
two successive days are exactly alike. The weather of 
a place near the seashore is different from that in the 
interior of the continents ; of the equatorial regions, from 
that of the higher latitudes ; of the mountain top, from the 
plain. Infinite variety is thus introduced, and while it 
will be impossible to state all of these differences, in the 
next few chapters Ave will point out some of the principles 
upon which they depend, and illustrate them by a few 
examples. 



CHAPTER IV 

LIGHT, ELECTRICITY, AND MAGNETISM 

Light 

Nature of Light. — When a bar of iron is placed in a 
fire, it becomes hot, and soon begins to glow; its black 
color is lost, and it becomes red or even white hot. It 
then gives out light and heat, and we are able either to 
read by its light, or to warm our hands by holding them 
over it. Like the iron, the sun is a very hot body which 
shines with a fiery light. 

If we place a white-hot bar of iron at the end of a room, 
it can be seen from the opposite side, and we may even 
be able to see it at a distance of half a mile. Something 
comes from the iron, which upon reaching our eyes, pro- 
duces there the sensation of light. The hypothesis which 
physicists have for explaining this, is the undulatory theory 
of lights according to which it is believed, that a series of 
undulations, or waves, are started in an invisible, and to 
us entirely mysterious substance, called ether^ which per- 
vades all space. These waves are thought to be somewhat 
like those in water, but they travel at the almost incred- 
ibly rapid rate of about 180,000 miles a second. So if a 
lamp is lighted at a distance of a mile, we perceive it 
almost at the same instant. 

43 



44 FIRST BOOK OF PHYSICAL GEOGRAPHY 

The hot solar orb is at all times emitting this radiant 
energy (light and heat) into space in all directions. So 
rapid is the movement of the rays, that the light and heat 
travel across space to the earth, over a path of more than 
92,800,000 miles, in a little over 8 minutes. Only a very 
small part of the light and heat from the sun reaches us 
(Fig. 12) and most of it goes out into space, vrhere, so far 
as we know, it is lost.^ Just as warmth and light from 
the hot bar of iron diminish as the distance from it is 
increased, so the heat and light of the sun lose intensity, 
until at the planet Neptune their amount must be slight. 

According to this theory, light is not a simple wave, but 
a complex series moving with slightly different wave 
lengths, and upon reaching the eye they together produce 
the phenomenon which we call white liglit.'^ Sometimes, 
and by various causes, this white light has some of its 
waves removed, and then we obtain color. So while light 
is ordinarily white, the sky is blue, the sunsets red or 
yellow, the leaves of trees green, and the flowers vari- 
colored. The difference in color depends upon various 
conditions, some of which may be simply stated, while 
others cannot be discussed in this book. 

Reflection. — During a part of the month the moon 
shines brightly in the heavens with a silvery white light, 
and again it disappears ; when the moon is young or old, 
we see the bright crescent, but the dull remainder of the 

1 It is somewhat as if we compared a lamp in a room to the sun, and a 
speck of dust on the wall to the earth. 

2 White light is made of a number of waves each having a color of its 
own. We recognize only seven of these, called primary colon's, — violet, 
indigo, blue, green, yellow, orange, and red, — the colors of the spectrum, 
which we see in a rainbow. This may be illustrated by a prism, which 
breaks up the white light into its component colors. 



LIGHT 45 

sphere does not shine brightly. Like the earth, the moon 
intercepts some of the sun's rays, and at times these are 
reflected to the earth, just as we may cause a sunbeam to 
be reflected from a mirror to some place which the sun's 
rays do not reach directly. On the moon the earth appears 
as a brightly illuminated orb in the sky. 

Gases do not readily reflect, but instead, allow the light 
to pass easily through them, being transparent ; but a 
water surface does reflect readily, and as the sun's rays 
reach the surface of a water body, we find them reflected 
from this in the form of a dazzling beam of light. ^ So, 
too, when the sun's rays reach a dense cloud bank, the 
water or snow particles which compose the cloud may 
reflect the sunlight ; and while the great mass of the cloud 
appears black or leaden gray, the edge upon which the 
sunbeams strike, becomes transformed to dazzling white. 
If the sunlight is colored, because of some change in the 
light rays, such as those explained below, the reflected 
light may he colored. 

Pure air of uniform density does not of itself reflect the 
light; but when a beam of sunlight passes through a gap 
in a cloud, its path to the earth may be seen, and the sun 
is then said to be ^^ drawing water. ''^ This appearance 
must be the result of reflection of light, for otherwise the 
distant rays would not be visible. If the air did not 
reflect light, the illumination of the earth in the daytime 
would be very different. The places in shadow would be 
very dark, if light were not reflected to them from the air 
and from the land. By means of a mirror, we may reflect 
enough sunlight upon a shadow to entirely destroy it; 

1 By an experiment with a dish of water, or a mirror, placed in the 
sunlight, the teacher can make the subject of reflection plain. 



46 FIRST BOOK OF PHYSICAL GEOGRAPHY 

and in a less complete way the natural reflection from 
many surrounding objects is engaged in the partial 
destruction of shadows. 

Shadows are less intense on hazy than on very clear 
days ; for then there are many solid particles in the air ; 
and it is these dust particles, rather than the air itself, 
which cause the reflection which is seen when the sun 
is "drawing water." In addition to the reflection from 
the dust, there is sometimes reflection from the air itself. 
This is because the atmosphere varies in density. We 
may see this on any hot, close summer day, when walking 
along a road or on a railway track. The warm air is 
set in motion, and the little currents thus started differ 
in density, and as they reflect light from their surface we 
see them dancing about. Each different layer acts some- 
what like a mirror. 

It is upon this principle that the mirage is caused. 
The traveller on the desert often fancies that he sees a 
sheet of water when he looks down upon the air that is 
warm near the surface, so that it has different density 
from that above, causing it to reflect light. On a lake or 
seashore the mirage lifts the distant shores above the water 
level, and ships ma}^ be seen apparently sailing in the air. 
When there is a warm layer of air above the surface, reflec- 
tions may be caused from it, and objects may then appear 
inverted, so that a ship may sometimes be seen apparently 
sailing in the heavens with its masts pointed toward the 
earth. 

In the Arctic regions, during the summer, the mirage is very com- 
mon ; and when sailing in the midst of floating ice, the effect of the 
mirage in raising the ice floe above the water level, often transforms 
the broken ice surface into a marvellously complex and beautiful 



LIGHT 47 

series of white imitations of cities, castles, and turrets. A single 
piece of ice is sometimes duplicated, until four or five pieces appear 
one above the other. Sometimes these join, forming a column; or, 
when partially joined, a sculptured turret; and as one looks upon 
such a scene, there is constant variation, and no two times is the same 
view seen in the same direction. 

Absorption. — Light rays pass easily through some sub- 
stances, which are then said to be transparent. Nearly 
all gases are transparent, or nearly so (then called trans- 
lucent'), and many liquids also, allow light to pass easily 
through them ; but solid substances are more rarely trans- 
parent. An instance of a transparent solid is glass, the 
transparency of which serves us so well in our windows. 

When light encounters bodies, even those that are most 
transparent absorb some of the light, while the remainder is 
either reflected or allowed to pass on iiito the substance. 
Those solids and liquids which do not allow light to pass 
easily through, and which are then called opaque, absorb 
most of that which encounters them. When very little 
of the light is absorbed, the object is white in color ; when 
nearly all is absorbed, it is black. But as white light is a 
complex of many Avaves, each producing separate colors, it 
happens that many objects allow some of the rays to pass, 
while others are reflected, and then the sensation of color 
is produced. If green is reflected in excess of the others, 
the color is green ; if more red is reflected than others, a 
red is produced, etc. 

This absorption of sunlight is important in the economy 
of life, particularly of many forms of plant life. While 
a potato will sprout and grow in a cellar, it does not pro- 
duce fresh green leaves, but instead, a sickly, yellowish- 
green stalk and leaves. Even men who dwell away from 
the sunlight lose their freshness. 



48 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Selective Scattering. — The light of the sun is probably 
bluish when it enters the upper layers of the atmosphere, 
becoming white in its passage through the air. In this 
passage, light rays suffer much change, a change which is 
not at all times the same. Sometimes the sky is a deep 
azure blue, again it is a pale, almost colorless blue, and 
there have been times when its color was a brassy yellow. 

Light waves, which are of various lengths, when pass- 
ing through air that is impure, find their progress par- 
tially checked. The coarser waves of yellow and red 
light are less easily disturbed than those having a small 
wave length, like the violet and blue. This may be com- 
pared to the ripples on a lake, which may be checked in 
their motion by a small sand spit, while the larger storm 
waves break over the obstacle. 

By this interference^ some of the rays are turned to one 
side and scattered. Since the waves that have small wave 
length are more easily turned aside, the violets and blues 
of the white light are scattered even if the air is very 
clear. Hence the inten Atj of the blueness of the sky is 
greater on particularly clear than on hazy days, when the 
scattered blue rays are partially obscured by the scattering 
of the other and coarser rays. When there is much dust 
in the air, even the yellow may be scattered ; and these, 
being then more intense than the blue, give to the sky a 
yelloAvish tinge. The color of the sky therefore depends 
upon which of the rays are scattered ; and since certain 
waves are selected^ according to the obstacle encountered, 
the process is called selective scattering. 

Refraction. — While there are several other peculiarities 
of light, some of which are important, only one more can 
be easily explained in this book. When a stick is placed 



LIGHT 49 

in a quiet body of water, with a part extending above the 
surface, it appears broken at the water surface. ^ This is 
due to refraction, the light ray itself being bent as it 
passes into the denser medium. If we could look from 
the water to the air, the stick would still appear broken, 
but would incline in the opposite direction. 

This bending of the rays affects those colors which have 
the shorter wave lengths, in a different way from those 
with the longer wave lengths^ So if we allow a sunbeam 
to pass through a glass prism, refraction bends the rays 
and affects the various colors differently. Hence, when a 
light ray emerges from a prism, instead of all the rays 
combining to cause white, the colors of the spectrum are 
thrown upon the floor, and we are then able to recog- 
nize the seven primary colors mentioned above. Many of 
our atmospheric colors, especially those of sunset, depend 
in part upon this principle of refraction of light rays, in 
passing through substances of different density. 

The Colors of Sunrise and Sunset. — When the sun is 
setting, its brilliancy is usually so decreased that we may 
look directly at it ; and if the air is dusty, it may be a great 
red orb, because the more delicate light waves have been 
scattered in passing through the great thickness of dust- 
filled air (Fig. 21). As the sun disappears, a glow of 
yellow, or red, overspreads the sky in the west, because, 
in addition to the blue light waves, the coarser reds and 
yellows have been scattered in passing through the mote- 
filled air. Those rays which come from near the horizon 
are most destroyed, and hence the coarsest of all prevail 
there, giving red colors, while those above the horizon, 
passing through less air, are yellow. 

1 This is an experiment whicli any pupil can try for himself. 



50 FIRST BOOK OF PHYSICAL GEOGRAPHY 

These solar rays are bent, or refracted, in passing through 
the dusty air, and when we see the sun just beginning to 
descend behind the hills, it is really below the horizon. 
This refraction lengthens the zone of coloring, so that the 
sunset colors often extend far on either side of the setting 
sun. In a clear sky the colors of sunset are arranged in a 
semicircular series, with the sun near the centre. At first 
the colors are yellow, fading out through tints of green to 
the sky-blue above. The yellow changes to red near the 
horizon. A second fainter series of colors, the afterglow^ 
often illuminates the sky, after the first glow of sunset 
has faded. In reverse order the sunrise colors exhibit the 
same changes. 

At sunset there is a dehcate pink tint in the eastern sky, grading 
upward and downward into the blue, forming an arch, known as the 
twilight arch. This is the result of reflection of some of the scattered 
rays produced at sunset ; and the dark or blue color, below the re- 
flected pink, is the shadow of the earth cast against the sky. 

These are some of the normal effects of the setting sun 
in clear weather. An increase of dust in the air, at first 
increases the intensity of the coloring; but beyond a cer- 
tain point, the dust deadens the colors, so that in a very 
hazy sky the sunset colors are absent, because even the 
waves of red and yellow light are so scattered that they 
do not reach the eye. Very often, both at sunset and 
sunrise, the horizon is partly occupied by clouds, and the 
cloud particles reflect and refract the light rays, produc- 
ing a marvellously beautiful and varied series of shades 
and tints of reds, yellows, brilliant gold, and deep purple. 
Our most perfect sunsets are in a clear sky ; but the most 
beautiful come when the heavens are partly clouded. 



LIGHT 51 

The Rainbow. — Standing at the foot of Niagara Falls, 
one may often see a beautiful rainbow outlined in the 
spray that dashes into the air at the base of the mighty 
cataract. Though forming an arc of a smaller circle than 
that seen in the eastern sky after a summer thunder storm, 
it is the same in cause. In each case there are drops of 
water — spray in Niagara, and raindrops in the rainbow — 
through which the rays of the sun are passing; and as the 
rays enter and emerge from the water drops, they are 
refracted, just as in the case of the light passing through 
a prism of glass. This refraction separates the rays into 
the rainbow colors, and these are sent back to the eye by 
reflection from the drops of rain. 

The form of the rainbow is that of a segment of a circle, 
with the ends resting on the horizon ; and the extent of 
the arc depends upon the position of the sun, being small 
when' the sun is high in the heavens, and nearly a semi- 
circle when it is near the horizon. The colors of the 
rainbow are those of the spectrum, with the red outside, 
and at times a second bow is produced above the true 
rainbow. In this the red is on the inside. Each person 
sees a different rainbow; but all see the same general 
features, because the raindrops always act in the same 
way in refracting light. 

Halos and Coronas. — There are other pecuHar and exceptional effects 
of hght, only two of which will be briefly raentioned. The halo, 
or ring around the sun or moon, occurs when the upper air is over- 
spread with nearly transparent clouds, composed of particles of ice. 
Usually the halo is a ring of white light ; but when best developed it 
has the colors of the spectrum, with the red inside. Arctic explorers 
describe brilliant halos, for in these cold regions, the air often bears 
numerous ice crystals, and it is refraction of light passing through 
these, and reflected from their surface, that produces the halos. 



52 FinST BOOK OF PHYSICAL GEOGBAPBY 

When denser clouds partly obscure the sun, the interference of 
these with the rays of light, sometimes produces coronas, which 
are circles of colored light concentrically arranged, and usually 
of small diameter, with the red on the outside. At other times a 
bar of light sometimes extends from the sun, and at times a cross 
is formed by two such bars. In rare cases these bars occur at the 
same time with coronas, and the circles are then divided into foui 
segments. 

Sunlight Measurement. — Physicists have measured the velocity of 
light by various means ; and nearly all the phenomena of light have 
received a satisfactory explanation on the basis of the undulatory 
theory. With these measurements we are not concerned; but meteo- 
rologists sometimes study the intensity and duration of sunlight. The 
former may be obtained by means of the black bulb thermometer (p. 69), 
the latter by the sunshine recorder. This is a metal box so placed 
that from sunrise to sunset the sunlight shines into it through a hole. 
On the inside of the box a piece of photographic (or blue print) 
paper is placed, and the sunbeam, entering the hole, travels over this, 
thus marking its presence by a line that is continuous if the sun 
shines all day, and broken if interrupted by clouds. At night the 
photographic paper is taken out and developed, and thus a line is 
marked where the sun shone, while no line is present if the sun's 
rays were interrupted. A similar result may be obtained from the 
black bulb thermometer. 

By such means we learn that in 1892 the sun at Yuma, Arizona, 
shone for fully 80 % of all the time that it stood above the horizon ; 
at San Diego, California, 62 % ; at Salt Lake City, Utah, 57 % ; at 
Washington, D.C., 52%; at St. Louis, Missouri, 44%; at Eastport, 
Maine, 44%; at Buffalo, N^.Y., 40%. 



Electricity and Magnetism 

Lightning. — During thunder storms, and other violent 
disturbances of the air, an electric spark is often caused to 
pass from cloud to cloud, or from a cloud to the ground. 
Lightning is then produced, and the sound caused by the 



ELECTBICITY AND MAGNETISM 53 

passage, 1 as it echoes and reverberates among the clouds, 
causes the roar and crash of thunder. When thunder 
storms are at a distance, and often when they are below 
the horizon, a flash of lightning illuminates the distant 
sky, and we see heat lightning. It is possible that atmos- 
pheric electricity has some influence upon the formation 
of rain, though of this there is some doubt; and although 
producing some vivid effects in the form of lightning, it 
is not now recognized as an important feature of the air.^ 

Magnetism. — Every one is familiar with the common 
magnet, which is a magnetized piece of iron capable of 
attracting other particles of iron. The earth is a great 
magnet with two poles of attraction, one south of Aus- 
tralia, in the Antarctic region, the other on Boothia 
Island, north of Hudson's Bay, in the Arctic. These 
poles attract the needle of the compass, Avhich is a piece 
of magnetized iron, so that the north end of the needle 
points toward the north magnetic pole. The attractive 
force is beneath the earth's surface, so that near the pole 
the magnetic needle dips vertically toward the ground. 
This condition of terrestrial magnetism is exceedingly 
important, for it furnishes us an easy means of obtain- 
ing directions by compass. This subject calls for con- 
stant study, because the attractive force steadily varies, 
so that the pole is not always in the same place. 

Magnetic action is also present in the sun ; and on the 
earth we are able to detect this by means of delicate instru- 
ments. Sometimes this solar magnetism produces what 

1 By an electric spark from Leyden jars an imitation of lightning and 
thunder may be produced. An electric car furnishes frequent illustrations. 

2 Just how this electricity is generated, and why it appears, is not 
exactly understood. 



54 FIRST BOOK OF PHYSICAL GEOGRAPHY 

are known as magnetic storms, when electric apparatus is 
disturbed and the atmosphere seems to be under the influ- 
ence of magnetic action. There is some relation between 
solar magnetism and sun spots. 

At times the northern sky may become illuminated at 
night, by the strange light known as the Northern Lights, 
or Aurora Borealis. This is apparently some magnetic 
disturbance in the air, by which a faint light is produced. 
Usually colorless, the aurora sometimes assumes various 
tints, and a variety of form that at times is remarkable, 
now waving like a drapery, now shooting backward and 
forward with great rapidity, while always it is so dim 
that the stars shine through it. The Northern Lights 
are much more frequently and better developed near the 
magnetic pole than elsewhere, although they may often 
be seen in the United States. In some way this appears 
to be related to the magnetism of the earth itself. 

Although careful studies have been carried on for a 
long time, the question of magnetism is still far from 
settled; and when it is understood, the explanation may 
throw much light upon various questions. There is 
reason for believing that the north magnetic pole of the 
earth, and the magnetism of the sun, are among the causes 
which produce, or at least direct, the great rain storms 
which travel across the country, causing most of the rain 
that falls in the northern states. Here is one of the great 
unsolved problems with which Nature confronts us, and 
yet one whose influence is constantly present and impor- 
tant. Our sailors, surveyors, and map makers are making 
use of a force whose nature no one understands. 



CHAPTER V 

SUN'S HEATi 

Nature of Heat. — Like light, our supply of heat" comes 
directly from the sun, whence it is emitted in company 
with light. 

Some hot bodies, like a stove, do not usually produce 
the sensation of light on the eye, but if their heat is 
increased, light is finally produced. Bodies which reflect 
sunlight, such as a piece of white paper, do not become 
warm as quickly as those like black paper, which do not 
reflect light. However, there is an intimate connection 
between these phenomena of radiant energy (light and 
heat), which have the same origin, and are possibly 
merely different expressions of the same thing exactly. 

Heat, like light, travels to us across space as a series of 
waves, moving with exceedingly rapid vibrations. It is the 
great life giver, for it warms our sphere, and upon its 
presence all forms of life depend for existence. Like light, 
heat changes its behavior on reaching the earth, and there- 
fore, for an understanding of the distribution of solar heat 
over the globe, we must first learn something of its 
peculiarities. 

Reflection of Heat. — Most bodies that allow light to 
pass easily through them, offer as little resistance to heat. 

1 It is exceedingly important that the students thoroughly grasp the 
principles treated in this chapter. 

55 



56 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Hence radiant energy (both light and heat) enters and 
passes readily through the transparent atmosphere ; and if 
the temperature out of doors is zero, a thermometer in a 
room placed near a windoAV, in the direct rays of the sun, 
will record a much higher temperature than that of the air 
outside. Such bodies as air and glass, which are trans- 
parent to heat, are said to be diathermanous. 

As in the case of light, all bodies can reflect some heat, 
and we even obtain a small quantity of reflected heat from 
the moon. Smooth bodies, and those having a light color, 
reflect both heat and light better than others, and we ma}^ 
prove this at any time by standing in a shadow upon which 
the light and heat from a window are being reflected. 
Quarrymen working in pits partly enclosed by rock walls, 
notice the same thing on a hot summer day, when the 
walls of the quarry reflect heat, and add to that which 
falls directly upon them from the sun. It is partly for 
this reason also, that the streets of a city are hotter in 
summer than the open country; for then not only does 
heat fall directly upon the street, but some is reflected 
from the enclosing walls of buildings. So also we may 
become sunburned in summer, even though our face is 
kept in shadow by means of a broad hat. The reflected 
heat from the ground performs this work; and since a 
water surface reflects better than the ground, one becomes 
sunburned much more quickly when on the water than on 
the land. 

During their passage through the air, some of the heat 
rays are reflected and scattered by the dust particles that 
are present; but most of them pass on and reach the 
ground. Most of the light immediately escapes, and 
when the sun's rays are absent, darkness prevails; but 



SUN'S HEAT 57 

heat in part remains, and its effect is still felt, to some 
extent, even when the sun rises in the morning. ^ How- 
ever, a large percentage of the heat rays that reach the 
earth, pass directly away, being turned back by reflection ; 
and so, as far as the earth is concerned, these are lost in 
space. 

Absorption of Heat. — A portion of the heat is absorbed. 
Black bodies absorb so much of the sunlight that they 
return to the eyes no distinct color. Such bodies also 
absorb much of the heat that comes to them, and hence in 
summer the asphalt pavement of cities, or the black rocks 
of the country, become hot. If we place a piece of black 
cloth upon a snow bank, in such a position as to be 
exposed to the direct rays of the sun, its warmth, result- 
ing from absorption, will cause the snow beneath it to 
melt, and the cloth will sink into the snow even during 
cold weather ; but if a white cloth is used, much more of 
the radiant energy is reflected, and the cloth does not 
become nearly so warm. Hence the warming effect of the 
sun's rays varies greatly, not only with the location, but 
also with the material that is being warmed. 

Radiation of Heat. — When we receive heat from a stove, 
rays come to us that are being radiated from the iron. 
The heat that comes from the sun is similar radiant 
energy which this great body is emitting; for a heated 
body is able to give out its heat to cooler surrounding 
areas, until the temperature of the two is equalized. 
Hence the sun will continue to radiate heat into space 
in all directions, until its temperature is reduced to that 

1 This may be illustrated in this way : if the sun shines into a room, it 
becomes light and also is warmed ; close the blinds and the light ceases, 
but objects that were reached by the sunlight will still remain warm. 



68 FIRST BOOK OF PHYSICAL GEOGBAPHY 

of space: just as a stove in which the fire has gone 
out will continue to radiate heat into the room until 
its temperature is reduced to that of the surrounding 
air. 

The same is true of the heat which comes to the earth 
from the sun, and which stays upon the surface for awhile. 
Some of this is absorbed, and the ground is warmed ; but 
during all the time that this heat is coming to the earth, 
it is being sent away into space, some by reflection, some 
by dii^eet radiation ; and it passes through the atmosphere 
in a way similar tq that in which it entered. Even dur- 
ing the hottest summer days, radiation is in progress ; but 
the ground continues to warm through the day, because 
more heat is being absorbed than can be radiated. How- 
ever, as soon as the sun descends behind the western 
horizon, the supply is cut off, while radiation continues, 
and hence the ground becomes cooler, and continues to 
cool until the sun again rises. 

During the summer the days are longer than the nights, 
and more heat comes than can be radiated. Therefore the 
ground warms day after day; but in the winter, radiation 
is in excess of the supply, so that the ground becomes 
cooler and cooler, until the days have again perceptibly 
lengthened. In this way the earth disposes of its surplus 
heat, and hence the heat of one year is not greatly dif- 
ferent from that of the preceding. If it were not for 
this action of radiation, each year would witness an in- 
crease of heat ; and the air and earth would soon become 
intensely hot if all of the rays were not sent away. 

Radiation is extremely important in explaining many 
of the features of the air, and in later pages we will 
need to refer to it again and again. Some bodies radiate 



SUN'S HEAT 59 

much better than others: grass and leaves are better 
radiators than the bare ground, and the earth radiates 
more readily than water. Hence the sun's rays affect 
various parts of the earth in a different way, and here 
is another reason for variation in temperature of the 
earth's surface. 

Conduction of Heat. — If the end of a rod of copper, or 
even of iron, is placed in the fire, the end in contact with 
the fire becomes very hot, and soon the other end, which 
is entirely away from the source of heat, itself becomes 
warm, and after awhile so hot that it cannot be held. 
The heat of one end passes through the iron by conduction^ 
being transmitted from molecule to molecule. In the 
same way, the rays of the sun, which come in contact 
with only the veri/ surface of the land, have their heat 
gradually conducted down into the soil. But the ground 
is not so good a conductor as iron, and at a depth of 
three or four feet, the sun's rays produce little effect 
even in summer, while at a depth of ten or twenty feet, 
the influence of the sun is almost absent. Thus the 
sun warms only the veri/ surface of the land. Water 
and air are even poorer conductors; but the air which 
rests directly on the ground, is gradually warmed by 
contact with the warm earth, and by conduction this 
heat is slowly transmitted into the air to a slight dis- 
tance above the ground. 

Convection. — When a kettle of water is placed upon a 
stove, the iron bottom is warmed, and the heat is con- 
ducted from the iron into the water. Heat causes expan- 
sion, as any one may see by watching a blacksmith put 
an iron tire on a wheel. The iron is warmed and placed 
outside the wheel, which it fits very loosely; but as it 



60 FIRST BOOK OF PHYSICAL GEOGRAPHY 

cools by radiation and conduction, it contracts and binds 
the wood of the wheel firmly together. ^ 

The expansion of a liquid or a gas makes it less dense 
or lighter. Therefore when the water in the bottom of 
the kettle is warmed, it becomes lighter than the layers 
above. This is an unnatural condition, for the lightest 
things float, as oil or wood will float on water. As the 
warming proceeds, the light layers of lower water are 
forced to rise by the sinking of the heavy cold layers, 
which are drawn down by gravity. This causes a boiling 
or convection. In the same way the lower layers of the 
atmosphere are warmed by conduction from the ground, 
which has absorbed heat. These become lighter, and an 
atmospheric boiling or convection is inaugurated. 

Ordinarily this convection is quiet and unnoticeable; 
but on a dusty road, or a railway track, during a hot 
summer day, the boiling of the air may be actually seen, 
as the little rising currents of warm air reflect light to 
the eye. The air seems to be in violent motion, and 
sometimes this is enough to obscure objects at no great 
distance. By this process of convection the atmosphere 
is set in motion in a much larger way, and this action 
furnishes an explanation of many of the winds of the 
earth (Chapter VII). 

Heat on the Land. — A few words in summary will show 
how the land is warmed and cooled. During the daytime 
the earth absorbs heat, much in summer and relatively 
little in winter; and at all times it is radiating, though 

1 So also the warming of the rails of a track in summer causes them to 
expand, so that the joints fit together, while in the cold winter they con- 
tract and spread apart. Therefore in laying rails it is necessary to leave 
a small space for this movement. 



sun's heat 61 

at night, when no heat comes, the effect of radiation is 
most pronounced. Some of the heat is conducted below 
the surface, so that the upper layer of the ground is less 
excessively warmed than it would be if all remained where 
it fell. Moreover, the air that rests upon the surface 
becomes warm by conduction, and this also takes away 
some of the heat. If the air were immovable, very little 
would be thus carried away through so poor a conductor; 
but as soon as a layer of air near the ground is warmed, 
it rises, cooler air forcing it up and taking its place ; and 
hence much heat is thus removed and distributed from 
place to place. 

The warming of the land is more effective in some situations than 
in others. In the case of black and light-colored rocks, we have 
instances of two extremes. The warmth of the land also depends 
upon its outline. The hilltop, being more exposed to the wind, and 
being able to radiate heat through less air than that which covers the 
valley bottom, is cooler than the valley. The increased warmth of the 
valley also partly depends upon the fact that the sides reflect heat into 
the valley, and check radiation from it, while the hilltop is open to 
the sky, and can radiate heat in all directions. A valley facing toward 
the south, and hence exposed to the direct rays of the sun, becomes 
warmer than one facing toward the east or west, and hence in shadow 
during part of the day. 

Warming of the Ocean. — For various reasons, water 
increases in temperature much less rapidly than land. In 
the first place, heat rays are more readily reflected from 
the smooth and often glassy water surface. The heat that 
is absorbed, and causes a rise in temperature, also expands 
the water. Since it is a mobile liquid, it is then set in 
motion, and currents are caused as a result of the change in 
density thus produced. So, while on the land some places 
become warmer than others, on the level ocean there is a 



62 FIBST BOOK OF PHYSICAL GEOGBAPHT 

uniformity of conditions, the material being all alike, and j 
so movable that if one part becomes warmer than another, * 
the difference is quickly equalized by means of a current. 

There is still another reason why water temperature is 
less easily raised than that of the land. It takes much 
more heat to raise the temperature of a certain bulk of 
water one degree, than it does the same amount of earth -A 
or rock. Also, water can be evaporated^ and in doing * 
this, much heat energy is used; but this heat is not made 
apparent by an increase of temperature, and so is called 
latent heat or heat of evaporation. The vapor thus pro- 
duced rises into the air and passes away in the winds, so 
that when it finally changes back to liquid water, the 
latent heat, which then becomes apparent, may appear at 
some very distant point and warm the air, instead of the 
ocean where it fell. Hence much of the solar heat that 
enters the water is borne away in vapor. 

For these reasons water warms very slowly, and even 
in midsummer the sea breeze that blows upon the land 
from the ocean, is a cool, refreshing breath of air. At 
night, and in winter, the water cools to a less degree than 
the land, because radiation from its surface is less rapid. 
Therefore in day and summer the ocean is relatively cool, 
and in winter, and during the night, it is warmer than the 
land. Hence the climate of the ocean is equable, and one 
of slight change; and the influence of this uniformity 
is felt upon the land which borders the sea. Because of 
the same peculiarity, even lakes of small size influence 
the local climate perceptibly. 

Temperature of Highlands. — After a storm in the moun- 
tains, or even in a hilly country, one may often see that 
snow has fallen on the hill or mountain tops, while rain 



I 



SUN'S HEAT 



63 



fell in the valleys. If one should carry a thermometer 
on a journey to a mountain top, he would find the tem- 
perature steadily descending; and the same condition is 
noticed by balloonists who ascend high into the air. By 
such observations, it is proved that the temperature of the 



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Fig. 18. 

Ideal section of atmosphere in temperate and equatorial regions, showing 
temperature at the surface and at various elevations. 

air decreases with elevation above the sea. The rate 
varies somewhat, but on the average there is a decrease 
of about 1° for every 300 feet of elevation. Even in the 
hottest parts of the earth, mountains may rise high enough 
to reach the region of eternal snow. 

This cold is partly inherent in the thin upper layers, 
and it is partly due to distance from the warm earthy 
which causes the temperature of the air near the ground 
to rise. The low temperatures of mountain tops are in- 
tensified hy radiation^ for the direct rays of the sun reach 
the mountains, just as they do the lowlands, though after 
having passed through a thinner layer of atmosphere. 
This heat is rapidly radiated, because radiation proceeds 
with much greater ease through the thin layer of rarefied 



64 FIB ST BOOK OF PHYSICAL GEOGBAPHY 

upper atmosphere, than it does through the denser, dust- 
filled air near the sea level. Hence the nights and win- 
ters on mountain tops are intensely cold, partly because 
of the cold air which surrounds them, and partly be- 
cause of the easy radiation. 

Effect of Heat on the Air. — On a very clear day, the noon- 
day sun sends its rays through the air with little interrup- 
tion, and the larger share of those that enter, reach the 
ground. Being clear, radiation and reflection from the 
surface are also easy, and much heat goes directly back, 
while at night, if the same condition of the atmosphere 
exists, radiation continues to proceed rapidly. Thus on 
a midsummer day, with the sun shining brightly, the air 
may be refreshing both by day and night. The heat is 
not imprisoned nor entrapped. 

If the sky is overcast with clouds, the heat rays pass with 
difficulty, and because so little heat reaches the ground 
the day is liable to be cool. After such a day, although 
little sunlight has passed to the earth, the night time 
is usually not cold, because the cloud covering, which 
prevented rays from entering, also acts as a blanket, and 
prevents the escape by radiation, of the heat which had 
previously come to the earth. At such times the tem- 
perature of day and night may remain about the same, 
because little heat comes and little leaves. 

If the day is hazy, or if the air contains much vapor, 
and is "muggy," this, to a certain extent, checks the pas- 
sage of rays from the sun; for the foreign particles absorb 
some of the heat as it passes ; but much heat still reaches 
the earth, and the impure air acts also as a barrier to its 
outward passage by radiation. So on such days, the ground 
and air are warm, and the day oppressive. Since radiation 



StlN^S HEAT 65 

is partly checked, even the night time may offer no relief 
from the oppressive heat; and man and beast suffer, until 
finally relief comes when the air is again clear, and some 
of the excessive heat caa be radiated into space. 

An atmosphere composed of pure nitrogen and oxygen, 
would offer little resistance to the passage of heat rays, 
because these gases are very diathermanous ; and in this 
case the air would be vejy slightly warmed by the passage 
of sunlight through it. But water vapor and dust parti- 
cles exist in the air, and these intercept some of the heat 
and thus become warmed. This heat is to some extent 
imparted to the neighboring air by conduction. These 
foreign substances intercept both the direct rays from the 
sun and those radiated from the earth, so that there is a 
double cause for warming. 

However, the air receives its warmth mainly in an 
indirect way. On a hot summer day, a thermometer placed 
a foot from the ground registers a considerably higher 
temperature than one 10 feet above it ; for the lower laj^er 
is warmed by contact with the heated earth. So by con- 
duction the air temperature is raised, and then by convec- 
tion the warm lower layers rise, and the heat from the 
ground is distributed, just as the air of a room is warmed 
by a stove, or by a steam radiator. 

There are many differences in warmth of the air from 
place to place, from hill to valley, from land to ocean, 
from one kind of ground to another, and from one latitude 
to another. By means of these differences in temperature, 
the elastic air is set into motion, and directly or indirectly 
winds, clouds, and storms are caused. The air is there- 
fore a carrier of heat, and it is always at work equallizing 
the differences in temperature, and hence in density. 



66 FIBST BOOK OF PHYSICAL GEOGBAPRT 

If in their passage through the atmosphere the heat rays 
are checked by dust particles when the sun is nearly ver- 
tical, and the thickness of the air is least, we can easily 
understand that at night, or early in the morning, when the 
rays pass through so much more air (Fig. 21), their effect 
is greatly decreased. This is one of the reasons ^ why the 
afternoon sun of summer loses its power as it sets toward 
the west, and why the morning sun does not quickly 



100° 
95° 
90° 
85° 
80° 
75° 
70° 
65° 
60° 
55° 


NOON 


NOON 


NOON 


NOON 


NOON 


NOQi^; 


100° 

95° 

90^ 

85° 

80° 

75° 

70° 

65° 

60° 

55° 
































■^-V 




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-/- 


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vy 


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Fig. 19. 

Diagram showing change in temperature for six successive summer days, the 
temperatux-e rising in the morning and early afternoon, and then descend- 
ing until the sun again rises. 



warm the earth and air. It is also one of the reasons 
why, even at noonday, the winter sun does not generally 
warm the land; for then the sun is low in the southern 
heavens, and the amount of air through which the rays 
must pass is similar to that of the afternoon sun. 

Effect of Rotation. — By the earth's rotation, in most 
parts of the earth, the sun each day mounts higher and 
higher in the heavens, first warming the earth feebly, as 
its rays traverse the great thickness of lower dense and 
dusty air, then increasing in intensity until afternoon, 
and again losing intensity as the horizon is neared. 

1 Another is the different angle at which the rays reach the surface. 



sun's heat 67 

There is another important action of rotation which affects the 
moving currents of air and water. In the northern hemisphere it 
causes tliem to turn to the right, in the southern toward the left ; or 
in other words, if they are moving toward the Equator, they are 
turned toward the west ; if from the Equator, toward the east. The 
deflective influence is greatest near the poles and least near the Equa- 
tor. It varies also with the velocity of the moving current, being 
greatest with those that move most rapidly. We shall have occasion 
to call attention in several places to the effect of this right-hand and 
left-hand deflection of air and water currents.^ 

Effect of Revolution. — Every part of the earth has its 
temperature influenced by revolution, for this causes sea- 
sons (p. 17). At all times the belt near the Equator 
receives more heat than that near the poles, for in the 
latter the rays never reach the earth from overhead, while 
in the former the sun is never far from vertical at midday. 
The angle at which the sunlight reaches the earth during 
the polar summer, corresponds in a measure to that of the 
light which comes to us late in the afternoon; but in 
winter no sunlight reaches the polar regions. 

Because of the difference in- the amount of heat re- 
ceived, the earth is divided into five great zones : (1) the 
Tropical^ Equatorial^ or Torrid; (2) the North Temperate^ 
between the Arctic circle and the Tropic of Cancer; 
(3) the South Temperate^ between the Antarctic circle 
and the Tropic of Capricorn; (4) the Arctic or North 
Polar zone^ within the Arctic circle ; and (5) the Antarc- 
tic or South Polar zone. 

So the solar rays vary in effect from one latitude to 
another, decreasing toward the poles; but this variation 
is partly checked by the revolution, which in all other 

1 No attempt is made to explain this phenomenon in this book, for the 
explanation is difficult to give without the use of mathematics. 



68 FIBST BOOK OF PHYSICAL GEOGRAPHY 

zones than the Tropical, produces seasons of alternate 
warmth and cold. Ordinarily the sun rises and sets ; but 
within the Arctic and Antarctic circles the revolution of 
the earth, with its axis inclined, destroys the difference be- 
tween day and night during a part of the year. Even out- 
side of these zones, the relative length of day and night 
is caused to constantly vary, even as far as the Equator; 
and in our latitude we have long nights in one season and 
short nights in the opposite. In the United States the 
noonday sun of summer shines from a point near the 
zenith. Six months later it is far down in the southern 
heavens, and day by day these conditions change. 

From this statement it will be seen that the question 
of temperature distribution over the earth is a complex 
one, and that there must be many variations from day to 
night, from season to season, from one latitude to another, 
from mountain to plain, and from land to sea. But if we 
understand the principles which have been dwelt upon in 
this chapter, we may study and understand the variations 
in the temperature of the earth's surface. 

Temperature Measurement. — Various instruments are in use for 
determining the temperature of the air; but the ordinary me /'cwWaZ 
thermometer is the most common and useful. A glass tube, sealed 
at both ends, and terminating in a bulb at the base, contains mercury 
with a partial vacuum above. The principle of its action is that 
liquids expand with warmth and contract with cold. Hence an in- 
crease in air temperature causes the mercury to rise in the tube, and 
the lowering of the temperature makes it necessary for the mercury to 
sink. The liquid thread which rises and falls, is forced up and down 
by the expansion and contraction of mercury in a cistern or bulb. 

Either the glass tube, or the standard upon which it is mounted, is 
graduated into a scale, the one in ordinary use in English-speaking 
countries being the Fahrenheit, in which the freezing point is 32°, and 
the boiling point 212°. Certain temperatures are oare^fully deter- 



sun's heat 



69 



mined, and marked on the tube, and then the remainder of the 
scale is graduated. On the European continent, the Centigrade scale is 
commonly used, this having 0° for freezing point, and 100° for boiling. 
It is a more simple scale, and is coming into use in this country, ^ 
Since the degrees recorded by warm conditions are high, we speak of 
high temperatures as synonymous with warmth, and low temperatures 
with cold. Other liquids can be used, and alcohol is commonly em- 
ployed where very low temperatures are encountered, for then mer~ 
cury freezes and ceases 
to act, while alcohol 
does not. 

A thermometer is 
now made of metal, with 
a clock face over which 
a hand moves. The 
operation of this de- 
pends upon the expan- 
sion and contraction of 
metal strips, and this 
change is conveyed to 
the hand, which moves 

over a graduated dial. Such metal thermometers may be connected 
with a pen point, which presses against a moving paper run by clock- 
work. These thermographs automatically write a continuous record 
of the temperature changes, the time of which may be told because 
the paper is moved regularly by clockwork. 

There are thermometers constructed to register the highest tem- 
perature of the day (maximum thermometers), and others to register 
the lowest (minimum thermometers). The hlack bulb thermometer may 
be used to tell the intensity of the sun's rays, and also for the purpose of 
recording the amount of sunlight. The instrument consists of two 
thermometers, an ordinary one, and one with a bulb blackened by 
paint or lampblack. Both are exposed to the sunlight side by side, 
and since the blackened bulb absorbs more heat than that of the 
ordinary thermometer, its record of temperature is higher. 




Fig. 20. 
A thermograph. 



"^ To convert the Centigrade to the Fahrenheit scale, multiply by 1,8° 
and add 32°. 



CHAPTER VI 

TEMPERATURE OF THE EARTH'S SURFACE 

Day and Night Change : Daily Range. — When the sun 
rises above the horizon in the morning, the temperature of 




Diagram to show difference in amount of air {TS) passed through by rays 
reaching the earth's surface {SS), nearly vertically {BA) and obliquely 
iCA). u 

both land and air is increased, though from the effect of 
radiation during the preceding night, it takes some time 

for the sun's heat to warm 



TEMPERATURE 



them. The temperature 

continues to rise until 

mid-afternoon, and then, 

as the sun's rays reach 

the earth at a greater 

angle, and after passing 

through an increased 

Fig- 22. thickness of air (Fig. 21), 

the effect is lessened, and radiation exceeds the supply of 

heat. Then the temperature descends, slowly at first, 

70 



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so- 
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24" 

23* 
22* 
21" 

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TEMPEBATUBE OF THE EABTH' S SUBFACE 71 



then more and more rapidly as the sun sinks behind 
the western horizon (Fig. 22). Radiation proceeds to 
lower the temperature until the sun again rises, and 
therefore the coldest 
period is that just be- 
fore the sunrise. By 
this means we have a 
normal rise and fall 
of the temperature 
each day that the sun 
shines. But there are 
many causes which 
modify this normal 
change or range, so 
that it differs from 
place to place (Figs. 
23 and 24), and even 
in the same place, 
from time to time 
(Figs. 19, 26, and 
27). 

Change with the 
Seasons. — This curve 
or range is not the 
same in all latitudes, 
but varies with the 
position of the sun in 
the heavens. Within 
the tropics, at all sea- 
sons, the midday tem- 
perature is very high, 
and after sunset it 




Fig. 23. 

Summer (heavy line) and winter (dotted line) 
daily range of temperature for several places. 
(1) Arctic day ; (2) St. Vincent, Minnesota ; 
(3) Djarling, India; (4) Jacobabad, India; 
(5) Key West, Florida; (6) Galle, India. 
6 and 6, near the warm sea. 



72 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



does not fall low enough to produce cold nights. Withm 
the temperate zones a warm day may be followed by a frosty 
night. As the season changes, the daily range in temper- 
ature varies. During the summer, the normal change is 
from hot midday to cool night ; but in winter the change 
is from very cold night to cool or cold midday. In the 
polar zones, the summer day is cool, and since the sun 

does not set, the night 
temperature is but lit- 
tle lower. In the win- 
ter, the temperature 
of day and night may 
be exactly the same ; 
but between these two 
extremes, when the 
sun does rise and set, 
a cool midday is fol- 
lowed by a cold night. 
Effect of Land and 
Water. — There is also 
a change with altitude, 
for even at midday in 
summer, the tempera- 
ture of highlands and 
mountains is not high, 
and the nights are 
cool, because these 
rise into the cool, thin, upper layers of air, where radia- 
tion is always very rapid. In the winter similar change is 
noticed, though the temperatures are lower. 

Between the ocean and the inland, there is also a de- 
cided difference in the daily temperature changes (Figs. 




Fig. 24. 

Diagram to show influence of ocean on daily- 
temperature range in summer and winter. 



TEMPEBATUBE OF THE EABTR'S SUB FACE 73 



M 6 A.M. NOON 6 P.M. m| 


lOtf 
90° 

so' 

70° 






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23 and 24). The ocean warms slowly, and it does not 
cool rapidly, while the land warms and cools with much 
greater rapidity. So the daily temperature range is greater 
in the interior of continents, than on the ocean, or on land 
that is influenced by the neigh- 
boring sea. Desert lands, being 
covered by a blanket of cool 
dry air, are greatly warmed in 
the day and cooled at night, 
because the heat of the sun 
passes easily through this air, 
and is radiated with equal ease ; 
but humid lands are not subject 
to so much change, because they 
are blanketed by a vapor-laden 
air. Therefore, the daily tem- 
perature range of the Sahara is 
considerably greater than that 
of the tropical belt of heavy 
rains in northern Africa, south 
of the desert. In the latter the 

temperature of both day and night is high, while in the 
Sahara very hot days are followed by relatively cool 
nights (Fig. 25). 

Irregular Changes. — There are many reasons why the 
daily rise and fall of temperature may be checked. If the 
sky becomes cloudy at midday, the sun's heat is partly 
prevented from reaching the land, and the temperature 
may not continue to rise, or if it does, it rises very slowly. 
A cool breeze may check the rise of temperature, so that 
the hottest part of the day is reached before noon. Clouds 
at night, or a warm breeze, may prevent the temperature 



Fig. 25. 
Diagram to illustrate greater 
daily range in temperature 
in a dry desert climate {B) 
than in an equally hot, but 
humid, tropical land {A). 



74 



FIEST BOOK OF PHYSICAL GEOGRAPHY 



from descending as it should. A humid or muggy air 
causes a smaller range of temperature than a clear, dry air; 
and when humid conditions prevail, the day and night 



M NOON M NOON M NOON M NOON M NOON M NOON M NOON M NOON M 1 


90 
85 
80 
75 
70 
65 
60 
55 
50 


































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AUG. 20, 1895 


21 


22 


23 


2* 


26 


29 


AUG. 27. 189b 



Fig. 26. 

Diagram to show irregularities of the daily temperature range. Compare 

with Fig. 19. 

temperature may change very slightly. Again, a storm or 
a cold wave may so interfere with the daily range, that the 
temperature may rise at night and fall in the day. There- 
fore, in reality, only a portion of all the days exhibit the 



?i: 



is 



1% 



Fig. 27. 

Diagram to show regular daily range of temperature and irregularities due to 
various causes. Notice April 22d and 27th. 

normal rise and fall of temperature, because this range is 
checked or altered in many cases, and as the result of 
various causes (Figs. 26 and 27).^ 

Seasonal Temperature Change : Seasonal Range. — As in 
the case of day and night, the temperature of the year rises 

1 The teacher can use these diagrams to test the ability of the students 
to detect illustrations of the points discussed in the text. 



TEMPERATURE OF THE EARTH S SURFACE 



75 



and falls. The cold ground of winter begins to warm as the 
sun rises higher in the spring, and the rays reach the earth 
more nearly verticall}^ after passing through less air. As 
this continues, the ground and air, cooled during the pre- 
ceding winter, become warmer ; and then, even after mid- 
summer (or June 20), the temperature continues to rise, 
although ^he sun's rays reach the earth less vertically. 
Therefore, the warmest time of year does not coincide 





MH. 


FEB. 


MJ(R. 


APR. 


MAY 


JUNE 


JULY 


Aue. 


SEPt. 


OCT. 


NOV. 


DEC. 




























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60 
40 

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20° 
40° 

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New York 



Fig. 28. 
Seasonal temperature range at several places. Singapore, in tropical ocean= 

with midsummer, but occurs later. After that, the tem- 
perature decreases as the rays reach the earth less verti- 
cally, and the days become shorter. We have our coldest 
days in January, because radiation is in excess of the sup- 
ply of heat, and the ground and air continue to cool even 
after the shortest day (December 21). 

Taking the average of all the temperatures of all the 
days of the year, we find a gradual increase from winter 
to summer, followed by a decrease until the coldest part 
of winter. Such a curve, if made accurately, would be 
somewhat irregular, because there are cool spells in sum- 
mer and warm periods in winter; but in spite of these 



76 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



temporarj variations there is a progressive change. 
This average seasonal change or curve (Fig. 28) may 
vary a little from one year to another, but in any given 
place it is nearly the same from year to year, although, of 
course, there is much variety in the temperature conditions 
from day to day. But in various parts of the earth, there 

is a very decided differ- 
ence in the nature of 
this seasonal change, 
dependent upon lati- 
tude, altitude, and re- 
moteness from the sea.^ 
Influence of Latitude. 
— At the Equator the 
noonday sun is always 
hiofh in the heavens, and 
the days and nights are 
nearly equal in length. 
There is enough change 
in these respects to 
cause seasons ; but since 



JAN. 


feb.|mar.|apr. 


MAY 


JUNE 


JULY 


AUG. 


SEPT 


OCT. 


NOV. 


DEC. 
















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00 

c 
80 

70 

a 
60 

c 
50 

40 

30 

20 

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Fig. 29. 



Seasonal temiDerature range in several places 

(1) St. Vincent, Minnesota ; (2) New York the SUmmcr and win 
state ; (3) Yuma, Arizona ; (4) Key West 
Florida ; (5) Galle, India. 



ter temperatures are 
both high, the range 
from season to season is not great. Within the temperate 
zones the sun is high in the heavens in summer, and the 
days are long, while in winter the sun is near the horizon 
and the nights are long. Hence the winter is cold and 
the summer warm or even hot. 



1 It will furnish good practice to have the students look up the loca- 
tion, elevation, etc., of these places; and by this they w^ill better appre- 
ciate the meaning of the differences. 



TEMPERATURE OF THE EARTH'S SURFACE 77 

Near the poles, within the frigid zones, the sun rises 
high enough in the heavens to raise the temperature con- 
siderably in summer, for then the sun remains above the 
horizon for weeks, or even months ; but during the winter 
it stays below the horizon, and then radiation cools the 
land and causes very low temperatures. 

Influence of Altitude. — Altitude is almost as important 
in determining the seasonal temperature of a place, as lati- 
tude. Even near the Equator there may be a frigid 
climate, with perpetual snow on the highest mountain 
tops ; and in the temperate zones many of the mountains 
reach the height where snow can remain all the year 
round. In such places the summer temperatures are 
never high, while the winter is cold. Therefore in the 
same latitude the seasonal range may vary greatly with 
elevation above the sea. In ascending mountains the 
average temperature descends, and this change is suffi- 
cient to influence the growth of plant life, so that forests 
change in nature and then disappear (Fig. 85), just as they 
do on the way from temperate to polar latitudes. 

Influence of Land and Water. — Practically the same 
effect is produced by land and water upon seasonal range, 
as upon the daily change in temperature (Figs. 28, 29, and 
30). Even in the temperate latitudes, where there is so 
much difference between summer and winter, the water of 
the ocean does not become highly heated in summer, nor 
very much colder in winter. This is partly because the 
water does not warm or cool quickly, and partly because 
it is in constant motion. Currents of cold water from the 
Arctic, and of warm water from the Tropics, are con- 
stantly flowing in the north Atlantic, and influencing the 
temperature of the air. Therefore the seasonal range 



78 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



over the ocean is never so great as it is on the land. One 
may feel this moderating influence of the water even on 
the shores of a lake, and the influence of the ocean itself 
is felt to a considerable distance from the shore. 





JAN.. 


FEB. 


MAR. 


APRIL 


MAY 


JUNE 


JULY 


kXiO,. 


SEPT. 


OCT. 


NOV. 


DEC. 


60° 
50° 
40° 
3C° 
20° 










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Fig. 30. 

Influence of water on seasonal range of temperature. Summer temperature 
at Nantucket low, and winter temperature high — the reverse at North- 
ampton. Compare with Fig. 24. 

Climatic Zones. — Because of these peculiarities the 
earth may be divided into five great zones of different 
temperature conditions : (1) the warm Tropical helt^ in 
which the temperature is always high, and the range mod- 
erate ; (2) the Temperate zones (one north and one south 
of the Equator), where the temperature is high in summer 
and low in winter, and the range great, while the average 
temperature is moderate ; and (3) the Frigid zones (one 
about each pole), where the temperatures are always low 
and the range considerable. 

Each of these zones must be further subdivided. In 
each there are insular or oceanic climates, differing from 



120 140 160 180 160 140 120 100 80 







A 



-^::j 






\% 



CU- 



t 




TROPIC OF CAPRICORN 



> 



::sj 



u 



■^. 



ISOTHERMAL LINES 

showing tte Mean Temperature (Fahr.) 
of the Globe for 

JTJIiY 



/^ 



120 140 100 180 160 140 120 100 



Facing page 79. 



40 GO 80 100 120 140 




20 40 60 80 100 120 140 



B.D.S<:rv<lss, S.Y. 



TEMPERATURE OF THE EARTH'S SURFACE 79 

the inland or continental; and there are highland and 
mountain climates, differing from those of the lowlands. 
Hence there is no single feature of the earth, which by 
itself will determine the temperature conditions. Lati- 
tude produces the most decided influence, and as we go 
from Equator to pole we pass through regions in which 
the average temperature is decreasing. Still places on the 
same latitude do not necessarily have the same climate. 

This can be very well illustrated by following a parallel 
of latitude, such as the fortieth parallel, which passes near 
Philadelphia. Crossing the Atlantic, where the tempera- 
ture is moderate, it enters Portugal, where the climate is 
very warm, then across the plateau of Spain, a warm, dry, 
semi-arid country, and emerges upon the warm Mediterra- 
nean, crossing the southern part of Italy, below Naples, 
the place to which people go for the purpose of a moderate 
winter climate. It then crosses Greece and Asia Minor, 
after which it enters the dry, cold plateau region of central 
Asia, crossing China, near Pekin, and northern Japan. The 
parallel passes across the warm Pacific, enters the United 
States north of San Francisco, where the climate is very 
equable, and ascends the mountains and plateaus of the 
west, now passing over a cold mountain top, and again 
descending to a warm, dry plateau enclosed between the 
ranges. It then passes between Kansas and Nebraska and 
then into Missouri, central Illinois, Indiana, Ohio, and 
southern Pennsylvania. There is every gradation from the 
warm, almost tropical climate of the Mediterranean, to the 
frigid climates of the high plateaus and mountains. 
^ Isothermal Lines. — An isothermal line is one which 
extends through places having the same average tempera- 
ture. That is to say, all the temperatures observed during 



80 FIRST BOOK OF PHYSICAL GEOGRAPHY 

a month (for instance) are averaged to furnish the mean 
temperature for that month, and the places having the 
same average are connected by a line which we call an 
isotherm. In the same way the average temperatures of the 
year give a basis for the construction of isotherms for 
the year. A map in which the lines of equal temperature 
are drawn is called an isothermal chart. Since the tempera- 
ture varies from one place to another on the same latitude, 
the lines of equal temperature, or isotherms, do not pass 
around the earth parallel to the degrees of latitude, but 
in a very irregular manner. 

Examining the charts which illustrate this chapter, we 
may see how the average temperatures of the world are 
distributed. The warmest belts are near the Equator, and 
the coldest near the poles ; but the hottest part of the 
earth is not exactly at the Equator, but north of it. This 
zone of greatest heat may be called the Heat Equator^ and 
the reason why this is north of the geographic Equator, is 
that there is more land in the northern hemisphere than in 
the southern. This becomes warmer than the water, and 
hence the temperatures in the northern hemisphere are 
higher, on the average, than those in the southern. Of 
course, we know very little about the climate in the cold- 
est parts of the earth, and nothing whatever about the 
conditions at the poles ; and so it is impossible to say just 
where the very coldest places are, though the coldest 
known place, or the cold jpole of the earth, is in northern 
Siberia, near the Arctic circle. 

There is an average decrease in temperature from the 
warm equatorial region toward the poles, and therefore 
the isotherms extend around the earth, following the direc- 
tion of the parallels of latitude in -a general way; but 




Facing page 80. 




S.D.Strvo.,,X.r. 




J^'acing page 81. 



60 80 100 120 




20 40 60 80 100 120 140 



S.D.Strt^aa, N.T, 



TEMPEnATUBE OF TBE EARTH'S STTRFACE 81 

they do so very irregularly, and this is particularly true 
in the northern hemisphere, where there is so much land, 
varying from plain to mountain, and separated by water 
bodies. In the southern hemisphere, where there is less 
land, the isotherms extend over the water surface in a 
direction nearly parallel to the lines of latitude. It will 
be instructive to follow one of these isotherms for each 
hemisphere ; and for this purpose we may select the iso- 
therm of 50° (the one which passes through all points 
whose average temperature for the year is 50°), which 
is the one passing through Boston, Mass. 

Crossing Massachusetts Bay to the tip end of Cape Cod 
(between 42°-43°), this isotherm extends northeastward 
fully 10°, crossing the middle of Ireland and England. 
It then descends to the Black Sea, crossing the northern 
end of the Caspian, and passing through central Asia near 
the parallel of 45°. It then descends below the 40th par- 
allel, crossing Corea and northern Japan at about 40°, and 
entering the Pacific. Then passing northeastward, the iso- 
therm enters this country near the mouth of the Colum- 
bia and passes into Canada, then again descends into the 
United States, passing along the southern portion of the 
Great Lakes. 

In the southern hemisphere this same isotherm passes 
around the earth nearly on the 40th parallel, generally being 
south of it, though sometimes passing to the northern side. 
Therefore in the northern hemisphere this isotherm ranges 
through about 15° of latitude, and in the southern hemi- 
sphere through not more than 8°. Some of the isotherms 
show even a greater difference than this. 

Comparing the January and July isothermal charts, we 
may see how much difference there is between the tern- 



82 FIBST BOOK OF PHYSICAL GEOGBAPHT 

perature of the coldest and the warmest months. In the 
north Atlantic the isotherm of 40° for January reaches 
above the 60th parallel, while in China it descends to the 
30th, ranging through more than 30° of latitude in the 
two regions. During July this isotherm is entirely within 
the Arctic circle, and in some places nearly reaches the 
80th parallel. In the southern hemisphere, during the 
southern summer (January), the 40° isotherm runs nearly 
parallel to and a little north of 60° south latitude, while 
in the southern Avinter it is just north of the 50th paral- 
lel. So the climate of the north temperate zone is shown 
to be more extreme than that of the oceanic southern 
hemisphere. 

The study of these charts reveals many other features. 
Where they flow, warm ocean currents raise the tempera- 
ture of the sea and bend the isotherms toward the poles ; 
a cold current, coming from the north, cools the air and 
bends the isotherms toward the Equator. Each of these 
features is very well shown in the north Atlantic on the 
January chart. A comparison of the charts shows how 
cold the interior of the continents becomes in winter, and 
how warm they are in summer ; and we find here an excel- 
lent illustration of the difference between the oceanic and 
inland climates. 

The world is so large that such maps as these can do no 
more than show the most general features. On those of 
the United States we may find other features, the result of 
minor causes, such as the influence of highlands in dis- 
turbing the direction of the isotherms. Thus on the 
Pacific slope the isotherms run nearly parallel to the 
coast, because the air, blowing in from the Pacific, rises 
against the mountain ranges and cools. These ranges 



TEMPEBATUBE OF THE EABTH' S SUBFACE 



83 



are nearly parallel to the coast, and so the isotherms 
run in this direction, each one representing a greater 
distance from the sea, and a greater elevation. If we 
could have even a more detailed chart of a small area of 
country, such as a state, we would find many other varia- 




FiG. 31. 

Isothermal chart of southern New England, showing influence of high and 

low land. 



tions. In New York, the Hudson and Mohawk valleys 
are warmer than the enclosing highlands, and the shores of 
the Great Lakes are more equable than the country which 
lies at a distance from these 'large water bodies. In New 
England, the Connecticut valley is warmer than the high- 



84 FIRST BOOK OF PHYSICAL GFOGBAFHY 

land region on either side (Fig. 31 1), and the same is 
shown in many other regions. 

Because of the many variations in heat effect described 
in the preceding pages, there are numerous kinds of cli- 
mate, — the warm and equable ocean, the heated desert, the 
interior plain, with extremes of heat and cold, the cold 
mountain tops, etc. There are also many other indirect 
effects. The sea is set in motion, winds are formed, vapor 
is taken from the water, rains are caused, storms arise, 
and the air is constantly doing work of various kinds. 

Temperature Extremes. — Owing to the irregular changes of 
weather, which are common in the temperate latitudes, we may have 
a great temperature change in a few hours. In Montana a daily i 
range of 50° is not uncommon, while at Key West, the difference j 
between the record of temperature in day and night is generally from j 
5° to 10°.2 In Montana the range between the highest temperature | 
of summer and the lowest of winter is about 145° ( — 40° in winter | 
and 105° in summer). At Key West the range is only about 40° j 
(50° in winter and 90° in summer). In Montana a fall of 100° has - 
been recorded in a few days, while at Key West there is never a great |, 
range. The one place has an insular climate, in the midst of a !' 
warm ocean current, the other is an elevated interior plain, far from i! 
the sea, and covered by relatively dry air, through which the sun's jj 
heat falls readily to the earth in summer, while in winter radiation ] 
proceeds with equal ease. Other parts of the country show ranges j/j 
between these two extremes.^ 

1 The teacher would do well to devote considerable time to the study 
of these charts, and to a discussion of the many features shown, fori 
which space cannot be given in this book. 

2 In the clear, dry region of Thibet a range of 90° in a single day is 
reported, 68° at midday and — 22° at night. 

3 The highest air temperature ever recorded is 127° in Algiers, and the 
lowest — 90° in central Siberia, which is the coldest known part of the 
earth. Higher temperatures than that of Algiers have been recorded 
from near the ground, for at times the earth of deserts, particularly if itsj 
color is dark, becomes so hot that it is painful to walk on the sand. 



CHAPTER VII 

'WINDS 

Air Pressure. — About us is a mass of air which presses 
down upon every part of the earth. At the sea level the 
pressure of the air amounts to about 15 pounds on every 
square inch of surface. A man therefore bears a weight 
of many thousands of pounds upon the outside of his body, 
but he moves about without realizing this, because the 
pressure is equal in all directions.^ 

The pressure of the air is not the same in all places, 
nor at all times (Fig. 32). For various reasons it changes 
from day to day, and is greater on cool, dry days than in 
stormy weather. The weight of the air also varies with 
altitude. While it amounts to about 15 pounds to the 
square inch near sea level, the pressure decreases as one 
ascends a mountain ; for of course there is less air over a 
mountain top than above a neighboring plain. It is found 
also that there is a difference in the air pressure even at 
sea level in different latitudes. The reason for this is 
explained below. 

Measurement of Air Pressure. — If -^e should fill with water a glass 
tube 35 feet long, having one end sealed, and then invert it with the 

1 One can prove the existence of this pressure, if he places his hand 
upon the top of a cylinder on an air pump, from which the air is then 
exhausted. The pressure is then removed from the under side, hut the 
weight of the atmospheric column above is felt, and the invisible load 
presses on the back of the hand with such force as to be painful. 

85 



86 FIRST BOOK OF PHYSICAL GEOGBAPHY 

open end in a dish of water, there would be a column of the liquid 
rising in the tube to the height of about 30 feet. This column of 
water represents the actual weight of an air column having the same 
area of cross section. Such a tube may be called a barometer, and if 
we watched the top of the water day by day, we should find that it 
rose and fell, indicating a change in air pressure. Since these changes 
are in reality detected by a similar instrument, it is common to speak 
of a change in barometer as synonymous with change in pressure (Fig. 32). 
When the air is heavy, the barometer rises, and we have a high barom- 
eter; when it is light, we have a loiu barometer. 











BAROMETER 
ITHACA 








9 






























7 
6 

: 

3 


























































"* N^ 














\ 






X 


^ 


^ 


^, 


1 

29 

9 

8 


\ 




^ 






\ — . 


/ 


^ 




/ 






N 


^ 


\ 




/ 






\^ 






^ ^ 


vy 
























6 
' 5 












































3 






























1 

?8 


















1 













10 



1892 

Fig. 32. 



16 



Diagram showing change of pressure for seven days. Figures on the side are 
inches and tenths of inches : —28.9, 29.1, etc. 

It is upon this same principle that the ordinary pump is constructed. 
A tube leads to the well, we exhaust the air from the tube by means 
of the handle, and then the air, pressing on the surface of the well, 
forces water into the tube. By such an arrangement we cannot pos- 
sibly draw water 40 feet above the surface of the well. A special kind 
of pump is necessary to raise water above the height of 25 or 30 feet. 

Such a barometer as that described above is too cumbersome for 



WINDS 



87 



use, and for it is substituted the mercurial barometer. The liquid mer- 
cury is very much heavier than water, and hence the weight of the 
air cannot force it so high in a tube which has a partial vacuum at 
the top. While water can be forced up to a height of about 30 feet, 
mercury is made to rise only about 30 inches. Any one can make a 
mercurial barometer by filling with mercury a glass tube 33 or 34 
inches long, with one end sealed, and then inverting it with the open 
end under the surface of a small dish of mercury. 

The ordinary barometer is made in exactly this way, although 
there are many changes in detail, in order to increase its perfection . 
One of these is the mode of reading the height of the barometer, or the 
elevation of the mercury column 
in the tube. The barometer is 
graduated in inches, and it is 
possible to read to tenths or even 
hundredths of an inch, so that 
very slight differences in air 
pressure are noticed. In speak- 
ing of the height of a barometer, 
we say that it reads 29.8, 30.1, 
etc., inches. A barometer at sea 
level has a higher reading than 
one at an elevation above the 
sea, and from day to day every 
barometer situated at one place 
is subject to change. 

Even the mercurial barometer 
is somewhat cumbersome, and is 
especially unfit for transporta- 
tion. When kept standing in 
one place it does very well, but 
it soon gets out of order when 
carried about. For some pur- 
poses it is important to transport 

barometers, especially when it is desired to measure the elevation of 
any part of the land. Since there is less air above a high land than 
above the sea, a barometer carried up a mountain side is subjected to 
less and less pressure, and the mercury correspondingly sinks a certain 




Fig. 33, 

Aneroid barometer graduated in feet 
(outside) and inches (inside). 



88 FIRST BOOK OF PHYSICAL GEOGRAPHY 

amount ; and hence for a fall of a fraction of an inch, we are able 
to calculate the amount of elevation over which we have passed. So 
the height of a mountain can be determined by means of the barometer. 
A change in pressure of an inch means an elevation of about 900 feet, 
though this varies somewhat with the amount of elevation. 

For this class of work a special form of barometer has been devised, 
called the aneroid (Fig. 33). Here instead of a liquid, the pressure 
of the air exerts its force on a metal diaphragm within a metal case. 
The change produced on the diaphragm is communicated to a hand 
which moves over the face, somewhat as the hands of a watch pass 
over the dial. The face beneath the hands is graduated to feet, so 
that as the index moves, a person may read the change. Just as he 
reads the time on a watch. In climbing an elevation, the hand 
moves in one direction, and in descending, in the other. The entire 
instrument is so small that it may be carried in the pocket. 

Aneroids are also used in recording a change in pressure. The 
instrument is placed in the proper position, and since the pressure 
varies from day to day, the hand moves backward and forward ac- 
cording to these changes. On it is fixed a pen which presses against 
a sheet of paper moved by clockwork (as in the thermograph). The 
pen therefore marks the changes in pressure, while the rate of move- 
ment of the paper keeps record of the time, so that a continuous 
record is kept both of the time and amount of change in air pressure. 
Such a self-recording barometer is called a barograph. 

Change in Air Pressure. — ^When air is warmed, the mole- 
cules are spread apart, and it becomes lighter. The same 
is true of a liquid. This can be proved by a simple exper- 
iment : place a drop of colored ice water in a glass of water 
having a temperature of 40°, 50°, or 60°, and the colored 
water will sink, because it is heavier than the remainder. 
Also on a cold winter's night, when the air outside is 
perfectly still, if a window in a warm room is opened, 
the heavy cold air from out of doors pours into the room. 
The same principle is illustrated by a stove or a lamp. 
The air near this is warmed, and hence made lighter than 



WINDS 89 

that which occupies the more remote parts of the room. 
The heavy air presses down and forces the warm air to 
rise, causing a circulation in the room. By standing on 
a chair in a well-warmed room, we may see that the 
cooler air stays at the bottom, while the warm air rises to 
the top. Oftentimes the upper layers are suffocatingly 
hot, while those near the floor are only comfortably warm.^ 

The atmosphere may be likened to the air of a room. 
Some places are being warmed more than others, and those 
that are most warmed are lighter than the colder portions. 
For instance, the air over the Mississippi valley may be 
warm, and that over New England cold, and we then have 
a high barometer, or heavy pressure, in the latter place, 
and a low barometer, or low pressure, in the former place. 
The barometer may register a difference of an inch in the 
two places. This difference in pressure gives rise to what 
is called the barometric gradient, and the air will move 
from the New England region toward the low-pressure 
area of the Mississippi valley, causing winds from the 
east. It does this because the atmospheric gases are so 
mobile that even slight differences in pressure cannot 
long exist. 

It is somewhat like pouring a glass of water into a 
basin. The water that is poured in does not stay in one 
place, raising the surface of that part of the basin, but 
flows about, and causes the neighboring layers to move, 
with the result that the entire surface is raised a little, 
and the pressure remains the same in all parts of the 
basin. Just so in the air: no sooner is a difference in 
pressure introduced than movement begins to equalize 

1 This subject of air circulation may well be prefaced by an experi- 
mental study, or a consideration of ventilation. 



90 FIRST BOOK OF PHYSICAL GEOGRAPHY 

it, and attempt to make the pressure everywhere the same. 
It is upon this principle that most of the winds of the 
globe depend. 

Planetary Winds : Theoretical Circulation. — The earth 
is divided into zones: there is cold about the poles and 
heat at the Equator. Therefore we may expect a great 
planetary circulation similar to that in a room containing 
a stove. Warmed most around the equatorial belt, the 
air here is expanded and made light, while on either side, 
north and south of this, there are belts with a cooler cli- 




'' ^ \ V o 



^S 



Fig. 34. 



Cross section of atmosphere showing, very diagrammat'cally, the general air 
circulation. E, equator ; S and N, south and north poles. 

mate. In the latter, since the air is hoavy, there must 
be a flow toward the warm region, in order to equalize 
the pressure produced by the difference in temperature. 
This will force the air near the Equator to rise, just as 
air ascends in a fireplace, or through the draft of a stove, 
or of a lamp. But if it rises, while air from the north 
and south flows in to become warmed and also rise, there 
must be some escape. Otherwise so much air will reach 



WINDS 91 

the Equator that the increase in bulk will make the press- 
ure as great as that in the colder temperate zones. 

The cause for this imaginary circulation of the air is at 
work every day in the year, and hence a movement of a 
very permanent nature must be begun. Again, we may 
make a comparison to a room containing a stove. Here 
the air is warmed near the stove, cold air flows in, forcing 
up the warm layers, which thus reach the ceiling and flow 
away to the sides of the room, while the air that moved 
in toward the stove is warmed and also made to rise. At 
the ceiling the air is cooled by contact with the cooler 
body ; it settles, and again flows toward the stove, so that 
a constant circulation is maintained. Here, then, we 
have four zones of movement : (1) the current along the 
floor toward the stove ; (2) the vertical current over and 
near it; (3) an upper current moving away from the stove, 
above that on the floor, and in the opposite direction ; and, 
finally, (4) the settling of the upper air on the opposite 
side of the room. Upon a much larger scale this is what 
we may expect to find on the earth. 

Trade -Wind Circulation. — Near the Equator there is a 
belt of calms^ which is the place where the air is rising. 
This ascending air cools, some of its vapor is condensed, 
and rain storms are of daily occurrence. It is therefore 
not only a belt of calm air, with light and variable winds, 
but also a very rainy belt, and it is sometimes called the 
doldrums. Moving toward the doldrums, on either side, 
are the trade winds, which flow over the ocean in a 
remarkably permanent manner, both winter and summer. 
These represent the cooler, dense layers near the earth, 
which are moving in toward the great terrestrial stove, 
the equatorial belt of calms. Above these, and flowing in 



92 FIRST BOOK OF PHYSICAL GEOGRAPHY 

an opposite direction, are the anti-trades, whose existence 
is proved by the movement of clouds high in the air, and 
also by actual observations on mountain tops, where peaks 
rise above the trade winds, and enter the upper currents. 
Near the tropics, at the place where the trades begin to 
be permanent, there are regions of settling air, known as 
the horse latitudes ; and thus the theoretical circulation is 
completed, and in fact we find what was predicted in theory. 

There is a very important difference, however: accord- 
ing to theory, the trades should blow southward in the 
northern hemisphere and northward in the southern, mov- 
ing directly toward the Equator, and the anti-trades should 
blow in the opposite direction. In reality this is not so; 
the trade winds of the northern hemisphere blow toward 
the southwest, while the. southern trades move toward the 
northwest, the northern anti-trades toward the northeast, 
and the southern toward the southeast. The reason for 
this is the influence of the earth's rotation (p. 67). Any 
current on the earth, whether of air or water, is turned to 
one side as it moves, to the right in the northern hemi- 
sphere and to the left in the southern. Hence the trades 
do not approach the Equator along the meridians. 

The belt of calms is not stationary, as is the Equator, 
but migrates with the seasons (compare Plates 8 and 9). 
During our summer, when the sun is vertical over the 
northern tropic, the belt moves to the northward; and in 
winter it migrates to the south, because the greatest effect 
of the sun's heat is first north and later south. Hence as 
the equatorial belt of greatest heat migrates, the trades 
change their position, in the summer being further north 
than in the winter. This change influences the climate 
of various places very perceptibly. 



1 



WINDS 91 

the Equator that the increase in bulk will make the press- 
ure as great as that in the colder temperate zones. 

The cause for this imaginary circulation of the air is at 
work every day in the year, and hence a movement of a 
very permanent nature must be begun. Again, we may 
make a comparison to a room containing a stove. Here 
the air is warmed near the stove, cold air flows in, forcing 
up the warm layers, which thus reach the ceiling and flow 
away to the sides of the room, while the air that moved 
in toward the stove is warmed and also made to rise. At 
the ceiling the air is cooled by contact with the cooler 
body; it settles, and again flows toward the stove, so that 
a constant circulation is maintained. Here, then, we 
have four zones of movement : (1) the current along the 
floor toward the stove ; (2) the vertical current over and 
near it ; (3) an upper current moving away from the stove, 
above that on the floor, and in the opposite direction ; and, 
finally, (4) the settling of the upper air on the opposite 
side of the room. Upon a much larger scale this is what 
we may expect to find on the earth. 

Trade-Wind Circulation. — Near the Equator there is a 
helt of calms., which is the place where the air is rising. 
This ascending air cools, some of its vapor is condensed, 
and rain storms are of daily occurrence. It is therefore 
not only a belt of calm air, with light and variable winds, 
but also a very rainy belt, and it is sometimes called the 
doldrums. Moving toward the doldrums, on either side, 
are the trade winds, which flow over the ocean in a 
remarkably permanent manner, both winter and summer. 
These represent the cooler, dense layers near the earth, 
which are moving in toward the great terrestrial stove, 
the equatorial belt of calms. Above these, and flowing in 



92 FIRST BOOK OF PHYSICAL GEOGRAPHY 

an opposite direction, are the anti-trades, whose existence 
is proved by the movement of clouds high in the air, and 
ateo by actual observations on mountain tops, where peaks 
rise above the trade winds, and enter the upper currents. 
Near the tropics, at the place where the trades begin to 
be permanent, there are regions of settling air, known as 
the horse latitudes ; and thus the theoretical circulation is 
completed, and in fact we find what was predicted in theory. 

There is a very important difference, however: accord- 
ing to theory, the trades should blow southward in the 
northern hemisphere and northward in the southern, mov- 
ing directly toward the Equator, and the anti-trades should 
blow in the opposite direction. In reality this is not so; 
the trade winds of the northern hemisphere blow toward 
the southwest, while the southern trades move toward the 
northwest, the northern anti-trades toward the northeast, 
and the southern toward the southeast. The reason for 
this is the influence of the earth's rotation (p. 67). Any 
current on the earth, whether of air or water, is turned to 
one side as it moves, to the right in the northern hemi- 
sphere and to the left in the southern. Hence the trades 
do not approach the Equator along the meridians. 

The belt of calms is not stationary, as is the Equator, 
but migrates with the seasons (compare Plates 8 and 9). 
During our summer, when the sun is vertical over the 
northern tropic, the belt moves to the northward; and in 
winter it migrates to the south, because the greatest effect 
of the sun's heat is first north and later south. Hence as 
the equatorial belt of greatest heat migrates, the trades 
change their position, in the summer being further north 
than in the winter. This change influences the climate 
of various places very perceptibly. 



WINDS 



93 



SOUTHERN HEMISPHERE 



Prevailing Westerlies. — Since the polar regions are 
places of greater cold, we might expect that in these zones 
the air pressure would be high; but such is not found to 
be the case, and we must look for some explanation. 
There certainly must be a settling of dense air in the high 
temperate and polar lati- 
tudes, because the cold of 
these regions makes the air 
heavy. We know that there 
is a rising of the air near 
the Equator, but the set- 
tling of this in the region 
of the horse latitudes does 
not take place so far north 
as this zone of greatest cold, 
but rather in the warm por- 
tions of the temperate zones, 
where the trade winds begin 
to blow. Here, however, 
only a part of the air settles, 
and some of the upper cur- 
rents, which extend as anti- 
trades from the place of 

equatorial upflow, pass along toward the poles, constantly 
turning more and more to the east, under the influence of 
the earth's rotation. 

West winds therefore prevail in the upper air of the 
high latitudes, and this is also the prevailing direction of 
the winds near the ground, though there are so many dis- 
turbing causes here that they are not so permanent as they 
are high up above the earth. Since both in the higher 
temperate and polar zones of the northern and southern 




Fig. 35. 

Ideal circulation of air near the sur- 
face, in the southern hemisphere. 
Trade : trade-wind belt ; H, H, horse 
latitudes of uncertain winds ; C.W., 
circumpolar whirl, or prevailing 
westerlies. 



94 FIRST BOOK OF PHYSICAL GEOGRAPHY 

hemispheres, the air movement, both aloft and near the 
ground, prevails from the west, these winds are called 
the prevailing westerlies. They complete the great planet- 
ary circulation of the atmosphere, and from them we 
obtain an explanation of the low pressure near the poles, 
where at first thought a high pressure would be expected. 

The air is moving toward the poles, where, because of 
the cold, there is greater density ; but as it comes from 
the broad temperate into the polar zones, it is passing 
toward a pointy the pole, and is constantly coming toward 
a narrower and narrower space. It would be impossible 
for all the air that starts to reach the polar zone, and hence 
some sinks and returns toward the Equator ; but much 
keeps on. A deflection results from the influence of rota- 
tion, which turns the currents to one side, and a whirl is 
begun, which is somewhat like that produced in water 
which is escaping from a basin. In this case the water is 
flowing from a broad area toward a narrow orifice, and if 
we look at such a whirl, we find that the water surface is 
lower in the centre than on the sides, and that around the 
depression the water is whirling in spiral currents. 

Conditions similar to this evidently prevail in the re- 
gion surrounding each pole, and the whirl of air, in the 
belt of prevailing westerlies, forms what is known as 
the eireumpolar whirl. Because of this spiral whirling 
movement, there is actually less air near the poles, and 
hence, notwithstanding the fact that it is colder and 
denser than in other parts of the world, its pressure is 
less than would be expected if it were not for the whirl. 

The great system of winds described above is better 
developed in the southern than in the northern hemi- 
sphere, because there is less land in the former. The 



WINDS 



95 



great expanse of water south of the Equator allows the 
air circulation to proceed with less interference than in 
the northern hemisphere, where bodies of land and water 
alternate (Plates 8 and 9). The heat of the sun affects 
the land much more than the water, and this difference 
causes other winds from water to land, or the reverse, 




Fig. 36. 

Map of Spanish peninsula, showing lines of pressure and temperature with 
winds for July, when the summer monsoon prevails. 



which mask, and at times even destroy the great planetary 
winds near the earth's surface. However, in both hemi- 
spheres the currents in the higher altitudes are remarkablj^ 
permanent, as may be seen in the United States by watch- 
ing the passage of clouds high in the air, which are 
generally moving from the west. 

Periodical Winds : Monsoons. — ^The effect of the sun's 
heat upon the land is greater than on the water, and radia- 



96 



FIRST BOOK OF PHYSICAL GEOGRAPHT 



tion from the land surface produces a greater effect than 
upon the water. Hence if there is a large land area, in 
summer it becomes warmer than the neighboring water, 
and in winter cooler. This may be seen by examining 
the map of the Spanish peninsula (Fig. 36). There, in 
summer, the air is less dense over the land than over the 
water; and, as in the case of the stove, a circulation must 
result. Therefore the denser air, which in summer lies 
over the Mediterranean and Atlantic, settles and forces 




Fig. 37. 
The summer (left hand) and winter (right hand) monsoons of India. 

the lighter air over Spain to rise ; but during the winter, 
when the land is colder than the surrounding water, the 
dense cold air over Spain settles and flows out toward the 
sea, causing winds in the opposite direction. These are 
monsoons, and they are periodical winds because they blow 
at certain definite periods of time. 

The typical monsoon is found in Asia, but it occurs also 
in Spain, Australia, Texas, and elsewhere. In Asia, par- 
ticularly over India (Fig. 37), the monsoon winds are re- 
markably permanent, and are important both in modifying 



WINDS 97 

the climate and in navigation. In the warm part of 
the year, the summer monsoon brings warm, damp air 
from the ocean, and heavy rains result; but the winter 
monsoon, blowing from an opposite direction, bears cold, 
dry air from the land. Year after year these changes of 
wind direction come as regularly as the seasons ; but there 
are of course other winds, due to different causes, which 
at times bring air from other directions. 

Land and Sea Breezes. — People go to the seashore to 
escape the heat of the summer, and they do this because 
the ocean does not become so warm as the land. This is 
very well illustrated in hot summer days, when the heat 
of the land is oppressive, and when, by going to the coast, 
one finds relief from the heat. Soon, if the day is quiet, 
a gentle breeze may be seen ruffling the surface of the 
sea (the same may be seen on large lakes, like the Great 
Lakes), and after awhile a cool draught comes from the 
water. This is the sea breeze, to which the dwellers by 
the seashore look for a relief from the oppressive heat of 
the summer day. It may reach a score or more of miles 
from the coast, but its chief effect is felt near the sea. 

At such a time the sea breeze may become a strong 
wind, and often in regions of permanent winds, such as 
the trades, a sea breeze may arise which succeeds in 
entirely changing the direction of air movement. Here, 
as in the monsoon, the cause is the heating of the land, so 
that the denser cold air of the sea flows in over the heated 
land. Before it begins to blow, the temperature by noon 
may have mounted into the nineties ; and then, with the 
coming of the breeze, the temperature may fall 10° or more 
in less than an hour, so that the time of greatest heat 
does not fall in the afternoon as usual. 



98 



FIRST BOOK OF PHYSICAL GFOGRAPHY 



At night, when the land cools by radiation, the dense 
air settles and flows out toward the warm sea, causing a 

land breeze^ which, 
however, is not so 
pronounced, nor so 
frequent in its occur- 
rence, as the sea 
breeze. The sea and 
land breezes are also 
felt along the shores 
of the greater lakes, 
though these are per- 
haps more properly 
called lake and land 
breezes. Even near 
some of the smaller 
lakes, similar air 
here only as slight 









^\ 


90 
80 


A'^\Sea Breeze \ 


// 


-•--4 



Fig. 38. 

To show effect of sea breeze upon the daily- 
temperature range. The normal range 
shown by heavy line, the influence of the 
sea breeze by the dotted line. 



noticed, though 



movements are 
draughts of air. This illustrates how easy it is for the 
atmosphere to move when the pressure varies slightly by 
differences in temperature. 



Mountain and Valley Breezes. — Among mountains, and even in 
hilly districts, the cooling of the air by radiation at night, causes a 
contraction of the lower layers, which becoming heavier, flow down 
hill toward the lower ground. Like water, the flowing air chooses 
the valleys down which to pass, and sometimes, in a valley having 
numerous branches, the breeze from the mountains and hills becomes 
very strong, and even increases to a gale before morning. 

In the daytime the warming of the mountain sides sometimes starts 
a reverse movement, which gives rise to a noticeable but less pronounced 
breeze up the mountain sides. Those dwelling in hilly regions may 
often feel the first-named wind during warm summer nights, but the 
breeze moving up the hillside is much less noticeable. 



WINDS 



99 



Irregular Winds. — There are other winds of less importance, and 
also a great group of winds having irregular directions, and associated 
in cause with extensive storms. These are the winds which are most 
pronounced in the United States, but their consideration inust be 
postponed until we understand something about storms (Ch. VIII). 

Velocity of the Wind. — The velocity of the wind is 
commonly stated in miles per hour, meaning the rate at 




// ///// A 



Ocsarv. 



Bluff. 




JtoUinc/ Ground. 



EUly Cau/nlry. 



Fig. 39. 



Diagram to show two of the several possible causes for the wave-like move- 
ment of the air. Here the air is disturbed in passing over hilly ground. 

which the air travels. A wind with a velocity of 10 miles 
is one in which the air would move 10 miles in an hour. 
A slight breeze has a velocity of from 2 to 10 miles, a 
strong wind from 20 to 30 miles, a gale from 40 to 50, or 
even very rarely 60 miles, while in a tornado (or what the 
newspapers call a "cyclone "), the velocity may be 100 or 
200 miles an hour. The rate at which the wind travels, 
varies with the difference in pressure, which the moving 



100 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



air is endeavoring to equalize; but this rate is retarded 
near the ground by the friction of air with the irregular 

earth. Hence 
on the tops of 
high towers, and 
especially of 
mountains, the 
wind blows with 
much greater 
force than it 
does on the 
ground; and it 
is also stronger 
over the smooth 
surface of the 
ocean than on 
the rough land. 




c a 

II 

o >, 

•S ^ 

«(-( .2 
O ^ 

>>.2 



"C ^ Recent studies 

® .« have shown that 

a ^ the wind is not a 

§3 simple onward 

J movement of a reg- 

^ ular kind, but a 

•9 series of pulsations, 

^ somewhat like the 

g puffs from an en- 

u erine. As the air 

be ° 

-J^ moves forward, it 

- also rises and falls ; 

and it has been found that even in times of strong w^nd there are 
momentary calms. We are all familiar with this in a larger way, 
when the wind comes in gusts; but besides these, which are well 
known and very noticeable, there are tiny gusts, so slight that deli- 
cate instruments are needed to detect them (Fig. 40). The vertical 



WINDS 



101 



movement of the air which accompanies this wave-like passage of the 
winds, is believed to be the motive power which such birds as con- 
dors and hawks use in their remarkable habit of soaring without the 
movement of their wings. Perhaps some day men may also make 
use of this principle in the con- 
struction of some air ship. 

Measurement of Winds. — 
Aside from certain delicate in- 
struments for special study of 
the wind, and from the result 
of wind studies in the higher 
air now being made by means 
of kites, there are two instru- 
ments commonly in use for 
studying the air movement. 
One of these is the ordinary 
wind vane, which tells the direc- 
tion. Every one is familiar 
with the construction of this 
and with its use. Sometimes, 
however, it is connected by 
electricity in such a way as to 
make an automatic record of 
changes in wind direction. 

Another wind instrument is the anemometer (Fig. 41), which is 
used for determining the rate at which the air moves. This instru- 
ment consists of four metal cups which are whirled about by the 
wind, and each revolution which they perform turns a cog wheel, 
which in turn moves others, causing a hand to move over a dial upon 
which are figures representing miles and fractions of miles. A 
certain number of revolutions of the instrument causes the hand to 
move over the space marked on the dial as one mile ; and therefore 
by reading this dial, one can tell how fast the wind blows, just as we 
may tell the time of day by the rate of movement of the hands over 
the dial of a clock. Oftentimes the instrument is connected by 
electric wire with a self-recording apparatus, and thus each revolution 
of the anemometer is automatically recorded. 




Fig. 41. 
An anemometer. 



CHAPTER VIII 

STORMS 

Weather Changes. — In the central and eastern states, 
there is a fairly regular succession of weather changes, 
though with many minor variations. A cool (cold in 
winter) spell of dry weather is followed by a rise in tem- 
perature which accompanies winds from southerly quarters. 
Gradually the sky becomes overcast, the wind changes 
toward the east, rain falls, and after awhile there is a 
clearing, with lower temperature, and wind from the north 
or northwest. During the summer this change may he 
preceded by thunder storms, and in the Mississippi valley 
by tornadoes. In winter it is often followed by severe 
cold weather, when a blanket of cold air overspreads all 
the eastern half of the country, possibly causing frosts 
even in Florida. 

Every five or six days this cycle is passed through, 
though perchance the rain may be slight, or may not be 
sufficiently widespread to affect the entire eastern region. 
Sometimes, particularly in winter, the changes in tem- 
perature are rapid and severe, and at times the force of 
the wind is great and its effect destructive, while at other 
times the winds are only breezes. These Aveather changes 
need to be studied in considerable detail. 

Weather Maps. — The United States government has in its em- 
ploy a corps of weather observers, stationed at various points in 

102 



STOBMS 



103 



the country, and furnished with thermometers, barometers, and 
other meteorological instruments, to be used in making observations 
on the changes in temperature, pressure, wind direction, wind force, 
etc. These observations, made at the same time of day at all stations, 
are telegraphed to headquarters, and the information thus obtained 
from all parts of the country, is placed upon a map, which is called 
the lueather map. These are printed and widely distributed, and 
any one sufficiently interested may obtain them. 




Fig. 42. 

Chart to show weather conditions January 7, 1893. Isobars (red) and red 
shading show the pressure, heaviest shading indicating highest pi'essure. 
Blue shows temperature, heaviest shade indicating lowest temperature. 
Areas of rainfall dotted. Arrows point in direction toward which the wind 
is blowing:. 



Upon the weather map are lines connecting places of equal tem- 
perature, or isothermal lines. By these one may tell' what the tem- 
perature has been in various parts of the country. Tsoharic lines, or 
lines connecting places having the same air or barometric pressure, 



104 



FIBST BOOK OF PHYSICAL GEOGRAPHY 



are also placed on the map. The pressure is marked in inches and 
tenths of an inch, thus : 30.4, 29.9, etc. Arrows point in the direction 
toward which the wind is blowing, and circles tell whether the 
weather is cloudy, rainy, snowy, or clear. A statement of the weather 
conditions is printed at the bottom of the map, and a prediction for 
the local weather of the nest day is also placed upon it. These maps 
contain much valuable information about the weather, but in some 
respects they are unsatisfactory, chiefly because the government does 
not have as many observers as are really needed for the work. 











Fig. 43. 

Chart to show weather conditions January 8, 1893. Shading, etc., same as 
Fig. 42. Path of storm centre shown by a series of arrows. 

Comparison of Weather Maps. — If we take a series of 
such maps for successive days, we are able to see the 
reason for the succession of weather changes noted above. 
A series of winter charts will probably best illustrate 
the points, for then the cjcle of change is most typical. 



ST0BM8 



105 



In the series selected for this description (and each 
winter will furnish many similar series), we start with 
one in which the word Low is placed in the Canadian 
northwest, north of Montana (Fig. 42). Around the 
word IjOW, the isobaric lines are arranged concentri- 
cally, the lowest pressure being within. Between the 



To - 3^ Y'^ 














1 !Atlan\S° "Vo dbarleston 

'jack3t.n,_ Jrontgomery 

^'ew Orleans^ 
Galveston 




Fig. 44. 

Chart to show weather conditions January 9, 1893. Shading, etc., same as 

Figs. 42 and 43. 



word Low and the northern boundary of Idaho the press- 
ure varies .4 of an inch, being 29.9 inches in the north, 
and 30.3 in the south, while further south the pressure 
is higher. Toward the area of low pressure the wind is 
blowing from various directions. 

In the east, near Nova Scotia, there is another Low^ and 



106 FIRST BOOK OF PHYSICAL GEOGBAPHT 

here, also, the isobars are arranged around the word, near 
which is the lowest pressure, of 29.3 inches. Toward 
this also the wind is blowing from all directions, and the 
weather in the neighborhood of the low pressure is rainy. 
The temperature here is between 10° and 20°, while that 
in the other low-pressure area varies from 10° to 40°. 

Twenty-four hours later, the more eastern word Low has 
disappeared, and if our map extended so far, we would 
find the conditions that caused it out on the Atlantic, 
beyond Newfoundland (Fig. 43). The western Low area 
has passed eastward to Lake Superior, and with it have 
gone the conditions of cloudy, rainy weather, and for the 
winter time, high temj)erature. In the meantime a High 
area, with clear weather and low temperature, has appeared 
in the northwest, near where we first found the low press- 
ure; and the next day (Fig. 44) the Low^ which is evi- 
dently a storm, has gone still further east, while the High 
has also moved eastward. A day later the Low is over 
the Bay of St. Lawrence, on its way out to sea, while the 
clear, cool weather accompanying the high-pressure area, 
has overspread New York and possibly New England. 
By this time another Low will have appeared in the north- 
west, and in this way, day by day, changes of similar 
nature are recorded by the weather maps.^ 

1 1 would urge upon teachers the advisability of obtaining various 
sets of such maps, which each student may study, so as to become 
thoroughly familiar with the facts illustrated, before undertaking the 
study of the nature of these changes. Also it is of high value to 
have the daily weather charts in the school. These will undoubtedly 
be sent regularly if application is made to the nearest Weather Bureau 
station, the location of which can be learned by addressing the chief 
of the Weather Bureau, United States Department of Agriculture, 
Washington, D.C. 



STOBMS 



107 



Cyclonic and Anticyclonic Areas : The Low- and High- 
Pressure Areas. — From these observations it is seen, that 
for some reason, an area in which the pressure is lower 
than the average, appears in the northwest, and progresses 
rapidly eastward, passing over the country in from two 
to four days. No case is known of such an area starting 
in the east and going westward, though at times they do 




Fig. 45. 

Map showing paths followed by low-pressure areas during November, 1891. 
Order of storms shown by Roman, and dates by Arabic numerals. Figure 1 
beneath the line indicates morning, and 2, evening. From these one can tell 
the direction and distance over which each storm travelled. 



begin in the southwest, and also in the West Indian 
region. When this is the case, the low-pressure area 
moves toward the northeast, and then across the Atlantic 
toward Europe, which they often reach. So we have as 
one universal fact, a path which ultimately leads toward 
the east (Fig. 45) ; and what is said of the low-pressure 
areas applies equally to the high. The most common path 



108 



FIRST BOOK OF PHYSICAL GEOGBAPHT 



for these disturbances of the air is eastward over the Great 
Lakes, through the St. Lawrence valley, over Newfound- 
land, and across the Atlantic, toward northern Europe. 

In their passage they sometimes die out, and on the 
other hand they at times rapidly develop renewed energy. 
In some cases there is only a slight difference in the 




Fig. 46. 

"Weather conditions April 20, 1893, showing two high-pressure areas and 
typical storm. Rain area shaded. 1.4 inch difference in pressure. 



pressure, while at other times the difference is great, and 
as these variations occur, there is a change in the velocity 
of the wind, which, when the difference in pressure is 
great, sometimes becomes very violent. As the low- 
pressure area progresses, the wind blows toward it from 
various directions, and when it is typically developed, 



STORMS 



109 



the air moves from all sides spirally toward the centre of 
lowest pressure (Fig. 46). Although the storm moves 
eastward, it is not to be inferred that the progress of the 
low-pressure area across the country is a bodily movement 
of the air ; for if this were so, there would be extremely 
violent winds from the west as it passed along. What is 
really the case in these disturbances of the air, is a low- 








1/ 






Atlanta" ^X ■t^^arleston 




Fig. 47. 

Map showing weather conditions November 27, 1896. From a high-pressure 
area in the west, the winds are blowing outward. In this high area the 
temperature is very low — temperature indicated by shading. 

pressure condition moving toward the east, just as a wave 
moves along the water surface with little real forward 
movement of the water. Hence the condition is this: 
an area of low pressure constantly progresses eastward 
with a wave-like movement, and toward this moving area 
of low barometer, winds blow from all directions. 



110 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



For the high-pressure area the same holds true, except- 
ing that here the air moves outward (Fig. 47). Other 
facts to be noted are, that the areas of high and low 23ress- 
ure, while sometimes circular (Fig. 46), are more often 
elongated or elliptical, and when this is the case, the long 
axis extends in a north and south direction (Fig. 48). 
In this case there is an even more notable resemblance to 




Fig. 48. 

Map showing weather conditions January 12, 1897. Two low and a high- 
pressure area with isobars extending nearly north and south. Most intense 
shading indicates highest j)ressure. Temperature in centre of high, 30 
below zero. 

a wave, especially when, as is sometimes the case, the 
trough of low pressure covers nearly the entire Missis- 
sippi valley, from north of the Canadian line to the Gulf. 
We need also to note the fact, that clouds, rain, and 
high temperatures generally accompany the low-pressure 
areas, and that they always do so if these are strongly 



STORMS 111 

developed, while the reverse is true for the areas of high 
barometer. 

Origin of the High- and Low-Pressure Areas. — It was once 
believed that all of these areas had their origin in differ- 
ences of temperature, much as in the case of the sea breeze. 
Such a cause would explain most of the features observed, 
because warmth would expand the air, and inaugurate a 
circulation, just as truly as it does in the case of the sea 
breeze, or the summer monsoon. As a result of studies 
made in Europe, doubt has recently been cast upon this 
theory. If the air is warmed, and is rising in the low- 
pressure areas, the temperature of the atmosphere on the 
mountain tops in such a centre should be warmer than 
that of the borders; but the studies mentioned, which 
were made among mountains, fail to show that this is 
true. Also the facts that these disturbances of the air are 
more pronounced in the coldest parts of the year, and that 
the areas of high and low pressure pass in such regular 
succession, are difficult to explain on this theory. There- 
fore, while such disturbances can be caused by differences 
of temperature, and while undoubtedly some are, many 
meteorologists believe that we must look for some other 
theory. 

No certain explanation can be offered in place of the 
old theory, but facts point toward the possible truth of 
the following. In the circumpolar whirl of prevailing 
westerlies, the air is moving eastward. If in its pas- 
sage it is thrown into waves, as the sea is, and as we 
may expect it to be in passing over the irregularities of 
the land (Fig. 39), troughs and crests of high and low 
pressure would be produced. These should come in a 
definite order, high following low, and with these varia- 



112 FIRST BOOK OF PHYSICAL GEOGRAPHY 

tions appearing at frequent intervals. This will also 
account for the eastAvard progress of the areas ; and which- 
ever theory is finally accepted, the explanation of the . 
movement toward the east, must be that the areas move in 
the great circumpolar whirl, and hence toward the east. 

Explanation of the Winds. — It has already been stated 
in sufficient detail, that air will move toward areas of low 

BROKEN CLOUDS CIRRUS CLOUDS 

CLEAR ^—.^^^^^^.^-rr^^^V.V /"^^ —^ ~-^-~^-^ ■' --r~' -?>^*'' 

.C'C_^ J-COLDE^ S^'^°''\^^{;C^ ^^W.RMER^" V\ 

HEAVY STRATUS CLOUDS 3^— ^EAST 

DIRECTION 

Fig. 49. ""^ "°vement 

Diagram showing theoretical movement of air (by arrows), and other condi- 
tions, in a low j)ressure or storm area. 

pressure, and away from areas of high barometer. Hence 
as such areas move across the country, the winds must 
blow toward a region of low barometer and away from 
that of high, in the attempt to equalize the pressure that 
has been disturbed. Since in the high-pressure areas, air 



CIRRUS CLOUDS 

CLEAR WEATHER 



EARTliay CLOU DV 



^.^\)\ A'l^i^/^Xrfe?^^" 






^ ^ 



y-^r^^\^-^^ 



Ds .^ m^ 



CALM DIRECTION 

OF MOVEMENT 

Fig. 50. 

Diagram showing theoretical circulation (by arrows) , and other conditions, in 
high pressure or anticyclonic area. Temperature rises on left. 

is moving outward from the centre, its place must be 
taken by other air which is pushing it onward. The 
source of this must be from above, for it cannot be from 
either side, since the movement is outward in all direc- 



STORMS 113 

tions. Therefore in high-pressure areas the air is settling 
from aloft. 

In the low-pressure regions, air moves toward a centre 
which is constantly shifting its position ; but as it comes 
from all sides toward the centre, some of it must find 
escape, and the only place for escape is upward. Hence 
here, there is ascending air, not perhaps of true convec- 
tional origin, but similar to that arising from convection. 
Perhaps the air that rises from the centre of the low- 
barometer area, passing upward, flows along toward the 
neighboring area of high barometer, and there settles, 
performing a journey similar to that in the trade-wind 
circulation (Figs. 49 and 50). 

Explanation of the Rain. — The subject of rain does not 
properly belong in this chapter, but on this point a few 
words must now be said. It is from the low-pressure areas 
that northern Europe and America obtain most of their 
rain supply, and these storms sometimes last for several 
days. In parts of New England these are called northeast 
storms, because the rainy winds of the storm are from the 
east and northeast. By meteorologists they are called 
cyclones, cyclonic storms, or extra-tropical cyclones. 
The diameter of the cloudy and rainy area may be more 
than 1000 miles; and as the storm moves eastward, the 
entire country, from the Rocky Mountains to the Atlantic, 
and from the Gulf states to Hudson's Bay, may receive 
rain, or in winter, snow. 

The causes for these rains are probably several. In the 
first place, the air is blowing in toward the low pressure, 
and as it does so it is often obliged to rise over mountains 
or plateaus, or up the more moderate grade of the interior 
plains. Since the temperature of the atmosphere decreases 



114 FIRST BOOK OF PHYSICAL GFOGRAPHY 

with elevation (Fig. 18), this lifting of the air over the 
rising ground causes it to cool. Also much of the air 
moves from a southern toward a cooler northern region, 
and in this way also its temperature is lowered. In a 
third way the air may become cooled as it rises in the area 
of low pressure. 

Vapor can be held in greater quantities when the tem- 
perature is high, than when low ; and therefore, if at the 
beginning of its movement toward the area of low barome- 
ter, the air was nearly saturated, it maiy, by being cooled 
in its passage, be forced to give up some of its vapor, 
forming clouds (Fig. 49), and rain or snow. This is 
particularly liable to haj^pen if the winds have come from 
the ocean, as they usually have when they blow from the 
south and east, toward the central and eastern states. 

To explain the dry, clear air of the high-pressure areas, 
or, as they are called, the anticyclones^ because of their 
contrast with the cyclones, we have but to reverse the 
statements just made. In these the air is settling, and 
hence becoming warmer; it is generally moving down 
grade ; and it is often flowing from cool to warm regions. 
Hence by all these causes the anticyclonic air is having 
its temperature raised, and therefore its capacity to take 
vapor increased. Instead of clouds and rain, such condi- 
tions bring clear and dry weather (Fig. 50). 

Explanation of the Temperatures. — When a cyclonic 
storm area is passing over northern United States, the 
winds of the country involved are usually from southerly 
or easterly quarters. These may come from the water, 
which in winter is warmer than the land, or from southern 
regions, which are also warmer. 

Hence the passage of air toward the low-pressure area 



STORMS lis 

is a cause for a rise in temperature. In addition to this, 
the cloud-covering checks radiation, and hence prevents 
nocturnal cooling. Also the condensation of vapor is a 
ivarming process^ the so-called latent heat, or the heat that 
is expended in transforming the water to vapor, is liber- 
ated when the clouds form, and raindrops are produced 
(p. 62). Hence even when a winter storm begins as a 
cold snowstorm, if the condensation of vapor continues, 
in time the weather moderates, and perhaps the snow- 
storm may end in rain. It is also true that this liberated 
heat furnishes energy to the storm, warming the air and 
making it lighter, thus decreasing the pressure and in- 
creasing the wind velocity and rainfall as well as the 
general intensity of the storm. The storm has become 
a great engine, which once started increases by the aid 
of fuel which it supplies to itself. 

The coolness of the high-pressure anticyclone, Avhich 
in winter may produce a cold wave^ and spread a blanket 
of ice-cold air over the land, is also due to several causes. 
The settling of the air from aloft brings down to the earth 
the high temperatures of the upper atmosphere (Fig. 50). 
Since the centre of the anticyclone is generally in the north, 
the air that flows over the United States usually comes 
from northerly quarters, and hence from colder lands. 
Since this air is dry and cool, radiation, both of day and 
night, proceeds with rapidity; and as a result of these 
several causes, the antic5^clone is distinctly cooler than 
the low-pressure area. When a well-developed anti- 
cyclone overspreads the northern states, the rapid radia- 
tion of night-time may cause even the summer night to be 
uncomfortably cool, while in spring, late frosts may come, 
or in the fall, vegetation may suffer from an early frost. 



116 



FIRST BOOK OF PHYSICAL GEOGBAPHY 



Hurricanes or Tropical Cyclones : Time and Place of 
Occurrence. — The sailors of the Atlantic believe that a 
violent storm may be expected at the autumn equinox, 
near the middle of September; and in reality, during 
August, September, and early October, the western 
Atlantic is liable to be visited by one or several very 
violent storms, which are called hurricanes or tropical 
cyclones. Similar storms visit the Indian Ocean and the 
south Pacific. The typhoon, which quite often devastates 




Fig. 51. 

A tropical cyclone in India, showing spirally inflowing winds toward area of 

low pressure. 

the Asiatic coast, is a similar disturbance of the air. Such 
storms are not known in the south Atlantic, nor do they 
ever originate on the land. They come in the autumn 
months (the autumn corresponding with our spring in the 
southern hemisphere), and are practically confined to these. 
Their birthplace is near the tropics over the ocean. 

The West Indian hurricanes have their origin between 




Facing page 116 



Plate 10. 



A tropical cyclone or West Indian hurricane. Path shown by line of arrows 
upon which the dates of passage are indicated. Winds, isobars, and isotherms 
shown. Rain indicated by shading. Path somewhat abnormal. 




B.V.Strvo».Ji.T. 



Facing page 117. Plate 11. 

Map of typical winter storm to show difference in temperature in different 
parts. Over dotted area rain is falling, over cross-shaded portion, near 
boundary between cyclone and anticyclone, snow is falling. 



STORMS 



117 



Florida and the South American coast, from which they 
move northward, with the centre generall}^ off our coast 
(Fig. 45), usually causing terrific gales from Texas to 
Nova Scotia. After passing along this coast, they cross 
the Atlantic, approximately along the path pursued by 
the cyclones, turning toward the east, under the influence 
of rotation, in exactly the same way that the trade winds 
are deflected. Sometimes these storms diverge from this 
path and pass over the land, so that the centre moves 
over the eastern states (Plate 10). 

Characteristics. — Where first noticed as distinct storms, 
the hurricanes are disturbances much smaller in area 



INCHES 

30.4 
30.3 
30.2 
30.1 
30.0 
29.9 
29.8 
29.7 
29.6 
29.5 
29.4 
29.3 
29.2 
29.1 
29.0 


SEPT. 29. 18^6.||SEPT. 30, 1896. 


OCTOBER 1 


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OCT. 3. 1896. 1 


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Fig. 52. 

Diagram showing sudden fall of barometer at Ithaca, N.Y., during the pas- 
sage of a hurricane, the winds of which did much damage. 

than the cyclonic storms which we have been considering. 
In the centre the pressure is very low, and the isobars are 
crowded together, so that in a short distance the pressure 
may change more than an inch. Toward this centre, the 
air goes from all sides with great force (blowing 60 or 70 



118 FIRST BOOK OF PHYSICAL GEOGRAPHY 

miles an hour), turning spirally as it moyes, somewhat 
as water does in escaping from a basin. Exactly in the 
centre, the air is rising vertically, and there the sky may 
be clear, while all around it are clouds, from which tor- 
rents of rain are falling. A vessel that has the misfortune 
to come within the reach of the more violent part of the 
hurricane, if it escapes at all, does so only after suffering 
much damage. Seacoast towns over which hurricanes 
pass, are often devastated, and this forms one of the most 
violent and destructive classes of storms. As they come 
out of the tropics, they gradually lose violence, and gen- 
erally at the same time increase in area. 

Explanation. — The origin of hurricanes seems evident from the 
fact that they always begin in warm regions. This points to convec- 
tion as their cause. Unusual heat brings about conditions as a result 
of which air must rise, and hence, upon cooling, furnish rain. The 
heat thus liberated by the condensed vapor, increases the ascent of 
the air by warming it, and this decreases the already low pressure. 
Toward this area of ascent, air comes from all sides, forming winds, 
which move spirally because they are deflected by the effect of rota- 
tion. The storm increases, slowly moves, and finally, passing into 
the cooler regions, loses intensity; for the cooler air that exists in 
the more northern latitudes, contains less vapor to supply the heat 
energy with which the intensity of the tropical storm is maintained. 
Such a storm could not be formed over the land, because the air is not 
so humid as over the water ; and hence the heat caused by the con- 
densation of vapor could not be supplied in such amount. 

But much heat reaches the tropical zone at all times of the year ; 
and why then do we not have such storms at all times ? and why are 
none found in the south Atlantic? To explain these two peculiari- 
ties, we must recall the fact that the deflective influence of rotation, 
which gives rise to the whirling movement of the air, decreases from 
polar to tropical latitudes, and near the Equator is so slight, that 
the air currents in the belt of calms cannot be turned very decidedly 
to one side. In the autumn the belt of greatest heat is furthest from 



I 



STORMS 



119 



the Equator, and hence nearest the region where this deflective effect 
can produce decided injiuence upon the direction of the winds. Hence 
at this time only, can the winds which blow toward the region of low 
pressure of the heated belt, start whirling ; and without the whirl the 
storm cannot exist. Since this heated belt never goes far south of the 
Equator in the south Atlantic, such storms cannot visit this ocean. 



M • NOON M NOON M NOON M NOON M NOON M NOON M NOON | 


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February 25. 1895 


.e 1 av 


28 


Ma.cn 1 


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Fig. 53. 

Temperature record for several successive days (Ithaca, N.Y.), showing effect 
of two cold waves, and (in centre) of Sirocco, which brought a high tem- 
perature for several days. 



Storm Winds. — While the greater part of the United 
States is within the belt of prevailing westerlies (Chapter 
VII), so that west winds prevail, the winds near the sur- 
face are mainly determined by the passage of the high- 
and low-pressure areas. The hot summer winds, which 
come from the south, generally represent air that is slowly 
flowing in toward a low-pressure area in the far north. 
They are often muggy winds because their source is from 
the warm, humid sea. Sometimes these winds blow day 
after day without cessation, and then the land may be 
visited by a summer drought. During the winter our 
ivarm winds are also from the southern quarter, and these 
likewise are moving toward a storm centre (Figs. 53 and 54). 
It is such winds as these which cause the winter thaws. 



120 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



In Europe similar warm winds are called siroccos^ and this 
name may also be applied to our own warm south winds. 

During the progress of a storm the wind in any particular 
place may veer through various quarters : the warm south 
wind may be followed by a warm and rainy southwest or 
southeast wind, and this by wind from the east, which 
bears rain, and this in turn by cold air moving from the 
northwest. The latter illustrates an exactly opposite type 
from that of the sirocco (Figs. 53 and 54). On the rear 
or west side of a storm, there are often strong and even 
fierce west and northwest winds, perhaps accompanied by 
snow (Plate 11). They represent cold air coming partly 



^ 



^^ 



^ 



Fig. 54. 

Temperature record (Ithaca, N.Y.) showing how for thirteen successive days 
the daily temperature range was destroyed by cyclones and anticyclones. 

from the upper layers of the atmosphere, and partly from 
the cool interior and more northerly regions. In Texas 
such a wind is called a norther, in Dakota a blizzard. 

Milder types of blizzards occur in New York, and other 
eastern states, after many of the winter storms. During 
such a time, perhaps after a rain storm, as the wind 
changes the temperature descends, perhaps even at mid- 
day, cold snow squalls occur, and soon the thermometer 
has fallen nearly to zero. After this comes the calm of 
the anticyclone, and the land, already covered with a 
blanket of cold, clear air, cools still more by radiation, 
iintil even lower temperatures occur. The cool, dry, and 



STORMS 



121 



refreshing west wind of summer, which succeeds the sultry 
weather that has perhaps terminated in a thunder shower, 
is the summer equivalent of the winter cold wave. 

Sometimes in the west, near the eastern base of the Rocky Moun- 
tains, in Montana and elsewhere, a wonderfully dry and warm wind 
springs up from the west, perhaps causing all the snow to disappear 
from the ground. This wind is known as the chinook (Fig. 55), and 



e..M. 12 (P.M. 


6..-. ,. 6..M. 1 




e..«. 12 9 P.M. 


9A.«. 12 9 P.M. 1 


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10 
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10 

80 


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Fig. 55. 

Temperature records during the blowing of the Chinook wind in Montana. 
In less than an hour the thermometer rose from 10° below zero nearly to 
40° above — a rise of nearly 50^. 

a wind of the same kind is known in Switzerland as the Foehn. Also 
in Greenland, the ah which flows down from the great ice and snow 
covered interior, is sometimes warm instead of cold, as we should ex- 
pect. These warm winds are caused by air blowing down the moun- 
tain slopes toward a centre of low pressure. When air settles rapidly, 
its temperature rises as a result of the compression, and hence it 
reaches the ground much warmer than when it started.^ The air is 
made dry, because as the temperature rises, its ability to carry vapor 
is increased. 

Thunder Storms. — During the summer, after an oppres- 
sive day, we look for a thunder storm, and it frequently 

iThis is a fact of physics, and is merely stated here as a fact, — that 
descending air has its temperature increased, and ascending air is cooled 
9,s it rises and expands. 



122 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



comes ; and often when one does not visit us, tliey occur 
to the north or south, and even sometimes so near that we 
see the lightning and even hear the thunder. In northern 
United States these storms come from the west, usually in 
the afternoon or early evening. Following the storm the 




Fig. 56. 
Photograph of a distant thunder storm. 



air is generally fresher, and cool, dry west winds replace 
the sultry air that has been coming from the south. 

Thunder storms are of frequent occurrence within the 
tropical belt. As the air is warmed by the morning sun, 
great banks of cloud begin to develop overhead, and finally 
rain falls, while lightning and thunder appear. Here the 
cause is evidently the rise of the air by convection ; for 
rising air cools, and if it started with much vapor, some is 
condensed to form clouds, and later rain, because with a 
lower temperature some of the vapor must be given up. 



STORMS 



123 



Around moTintain peaks similar storms are developed in 
summer, and here also convection is the cause. 

The same explanation appears to apply to the thunder 
storms of the United States, for these come only in the 
warm months, and when 
the air is very humid. 
Moreover, their coming 
is preceded by the for- 
mation of cloud banks, 
similar to those caused 
by convection in the 
tropics.^ If one will ex- 
amine the weather map 
for a day when thunder 
storms occur, he will 
find that they have de- 
veloped in the southern 
quarter of a low-press- 
ure area, where warm, 
humid south and south- 
east winds are blowing 
toward the storm centre 
(Fig. 57). They are 
therefore secondary 
storms^ occurring in a 
larger area of low pressure, and developed mainly because 
of the heat made possible by the south winds, and of 




Fig. 57. 

Part of weather map July 16, 1892, showing 
storm over easteru Canada and north- 
ern New England. Thunder storms 
occurred at places marked by arrows. 



1 That convection is the cause for these clouds is shown by the fact 
that they often form over the land, and not over the cooler ocean, where 
there is less convection. In sailing off shore, one often sees a line of these 
clouds while the sky overhead is clear ; and the presence of distant land, 
which is out of sight, is shown by these banks of clouds. 



124 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the moisture, which is due to the same cause. They move 
eastward in the same direction as the low-pressure area, 
and sometimes many such storms develop and progress 
eastward, following approximately parallel paths. Some 
such storms have travelled from New York across New 
England, and disappeared upon passing out to sea. 

Tornadoes. — Fortunately these terrible storms are uncommon in 
most of the country, and where they do occur, they extend over only 
a very small tract. As in the case of thunder storms, they develop in 
the southern part of low-pressure areas, and move eastward during 
the afternoon or evening of hot, muggy days, generally in summer. 
Seen from one side, they consist of a spout of black cloud, spreading 
out into an umbrella shape above (Fig. 58). Rain and hail fall from 
the margin, and lightning and thunder accompany the storm. Ex- 
cepting near the spout the wind is not violent ; but in this it attains 
such a velocity that strong buildings are torn apart, trees uprooted, 
heavy objects lifted and borne away, and many remarkable feats per- 
formed. 

The wind in a tornado blows spirally toward the centre of the 
spout, with increasing velocity until the centre is reached, where the 
air is rising rapidly enough to lift the roofs of houses from their sup- 
ports. Here no rain can fall, for everything so light must rise. In 
the centre the barometric pressure is extremely low, and the condition 
of a vacuum is so nearly reached, that the expansion of the air within 
the houses sometimes blows the walls outward. The tornado spout is 
somewhat like the whirl of water which escapes from the outlet of a 
wash basin. On a small but much more violent scale it is like a hur- 
ricane, and on a much larger, and also more violent scale it resembles 
the tiny dust whirls of the desert, which I shall describe. 

On a plain, and better still on deserts, the heat of the sun warms 
the air near the ground, until its temperature is several degrees higher 
than the layers above. This is an unstable and unnatural condition, 
for warm, light air should rise ; but the day is so quiet that for awhile 
nothing causes it to start. Then perhaps the flutter of a bird starts a 
movement which shall give relief to the unnatural arrangement of air 
layers. Air presses from all sides toward the place where the ascent 



STORMS 



125 



is being begun, and soon a slight whirl is started, and the movement 
of the air is so rapid that the winds carry dust, leaves, and even sticks, 
which in the centre rise, and spreading out above, fall to the ground 
on one side of the centre. From all directions the air moves toward 
this spout, and the little dust whirl itself moves slowly across the plain, 
so that if one should stand in its path, he would find the wind first in 
his back, when his hat would rise into the air, and quickly the wind 




Fig. 58. 
A tornado near St. Paul, Minnesota, July 13, 1890. 



would blow directly in his face, at first briskly, then more gently, 
until finally replaced by the calm of the desert. On a milder and 
very small scale this is what is experienced in a tornado. 

During days when tornadoes come, the air near the ground is very 
warm and humid, while cooler layers of air exist above. Convection 
causes a whirl to start, and a tornado develops, being no doubt in- 
creased in force by the formation of rain, which causes more heat. 
The reason why tornadoes are more abundant in the Mississippi val- 
ley than elsewhere, is that here, over the great plains, the warm air 
from the south is more easily drawn in undei^ the cool air, which is 
moving from the west in the prevailing westerlies. 



CHAPTER IX 

MOISTURE IN THE ATMOSPHERE 

Vapor. — This invisible form of water is always present 
in the air, and every now and then some of it is being 
changed from an invisible gas to the liquid or solid form 
of water. This substance finds its way into the air as 
the result of evaporation^ and at nearly all times vapor is 
being taken from all water bodies in the world, as it is 
also from damp ground and from the leaves of plants. It 
is possible for some vapor to be held in all air, no matter 
how cold, but there is a limit to the amount that air of 
any given temperature can hold. When the air has so 
much vapor that no more can be taken, it is said to be 
saturated; hence such humid or saturated air does not 
have the power to carry on evaporation further. On the 
other hand, an air that is dry, and not nearly saturated, is 
capable of rapid evaporation. Therefore in deserts, where 
the air is exceedingly dry, water cannot stand long with- 
out being evaporated. 

The rate of evaporation depends partly upon the dryness 
of the air, partly on its temperature, and partly on its 
movement. When the air remains quiet over a pond, it 
may become saturated, and hence for the time being, 
evaporation over the water surface may be checked; but 
if the wind is blowing, there are constant supplies of new 

126 



MOISTURE IN THE ATMOSPHERE 127 

air^ and hence evaporation proceeds without interruption. 
The winds that pass over the great ocean can obtain much 
vapor, and hence the air of oceans, as well as that of the 
land near them, is generally more humid than that far 
away in the interior of continents. 

In any given amount of air there is always a certain 
quantity of water vapor ; and this is known as the absolute 
humidity^ and could be measured iri pounds (Chapter III). 
If we suppose that air with a temperature of 60° has | as 
much vapor as it could possibly contain at that tempera- 
ture, this £ would be called the relative humidity ; that is, it 
would be the percentage of vapor actually present in the air, 
compared luith that 7uhich might he held at that temperature. 
If it contained all that could possibly be held, or was 
saturated, the relative humidity would be 100% ; hence 
the |, which we have supposed, would be 75% of the pos- 
sible, and the relative humidity would therefore be 75%. 
Should the temperature of this same air fall, even without 
the least change in the real amount of vapor, or the abso- 
lute humidity, the relative humidity Avould be increased, 
because cold air is able to carry less vapor than when 
warmer. Indeed, it is possible that the temperature may 
descend until the point of saturation is reached, and if 
this be so, some of the vapor must be given up either in 
the form of fog, rain, dew, frost, snow, or hail. This 
point of saturation, when the relative humidity stands at 
100%, is known as the dew point. 

If, instead of falling, the air temperature rises, the rela- 
tive humidity will decrease, because it can hold more vapor 
than formerly; and therefore, with a higher temperature, 
the percentage of that held compared with what might be 
carried is smaller. These facts have important bearings 



128 



FIRST BOOK OF PHYSICAL GEOGRAPHT 



on the explanation of the precipitation of moisture from 
the air. Evaporated and transformed to a gas which no 
one can see, the vapor passes hither and thither, until 
finally, by some change of temperature, it can no longer 
exist as vapor, but must assume its old form, and perhaps 
return to the very ocean which gave it birth. 

Almost any place in moist countries offers frequent 
illustrations of this change in relative humidity. Per- 



MONDAY 


TUESDAY 


WEDNESDAY 


THURSDAY 


FRIDAY 


SATURDAY 


SUNDAY 


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6 XII 6 


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AUG, 20, 1893 



RELATIVE HUMIDITY 
JTHACA 

Fig. 59. 

Record of relative humidity for a week at Ithaca, N.Y. Nearly every night 
the dew point is reached, but at midday the relative humidity is only from 
30°-60°. 



haps for several days the air of a place has about the same 
actual amount of vapor^ or the same absolute humidity, 
but the temperature of the air varies from day to night. 
As the sun's heat warms the earth in the daytime, the 
relative humidity of the air decreases, and perhaps by 
noon there is only 50% as much vapor as can be held at 
that temperature, and then evaporation is rapid, as may 
be seen by the fact that clothes on the line dry quickly. 
In the afternoon the temperature descends, and therefore 
the relative humidity rises, until perhaps the point of 



MOISTURE IN TEE ATMOSPHEBE 



129 



saturation is reached (Fig. 59), and then dew 
may form; and if clothes have been left out 
upon the line, they again begin to become 
damp, although before sunset they were dry. 
In addition to this daily change^ dependent 
entirely uj)on variation in temperature, there 
are also changes in the absolute humidity of 
the air, for some winds are damp and others 
dry. 



Instruments for Measuring Vapor. — Although we 
cannot see vapor, there are various ways in which we 
can measure the relative humidity. One of the commonest 
of these is the Tiair hygrometer, which consists of a bundle of 
hair robbed of its oil. The individual hairs absorb the 
vapor in proportion to its amount, and as they absorb or 
give up vapor they lengthen or contract. This movement 
of the hairs can be made to move a hand across a graduated 
scale, so that readings of the length may be made, and from 
this the relative humidity be calculated. The operation of 
this instrument is upon the same principle that causes hair 
not naturally curly to lo.se the artificial curl when exposed 
to damp air. This results from the absorption of vapor 
from the air. 

Another means for making this measurement is by the 
use of two thermometers, one having its bulb encased in a 
piece of wet muslin. This instrument, which is called a 
psycJirometer, is whirled in the air, and one of the thermom- 
eters records the real air temperature^ while the other, which 
has its bulb wrapped in wet muslin, records a loiver temper- 
ature, because the evaporation of water from the muslin 
produces cold, as evaporation always does.^ If the air is 



Fig. 60. 

Psychrometer or 
dry (right hand) 
and wet (left 
hand) bulb ther- 
mometer. 

K 



1 This principle is made use of in dry coun- 
tries to keep water cool. Water in a porous jar 
evaporates through the sides, thus cooling the jar 
and also the water. In travelling in such coun- 
tries it is customary to carry a canteen of tin, 



130 FIRST BOOK OF PHYSICAL GEOGRAPHY 

very dry, the evaporatiori. is rapid, if it is humid, the evaporation pro- 
ceeds slowly ; and hence in the former case the difference in temper- 
ature recorded by the two instruments, is greater than in the latter. 
By means of tables made for the purpose (these may be obtained 
from the U. S. Weather Bureau at Washington) the relative humid- 
ity may be calculated, and from them also it is possible to tell at 
just what temperature the dew point will be reached under all con- 
ditions of relative humidity, i 

The rate of evaporation is generally determined by means of a pan 
of water (called an evaporating dish) either placed upon scales, and 
hence weighed, or else one in which there is a graduated rod. By 
means of this rod the measurement of the depth of water evaporated 
is made in inches ; and hence, in stating the evaporation, it is custo- 
mary to say that it amomits to so many inches in a year, by this 
meaning in the course of a year that evaporation from a water body 
would lower it just that number of inches. In dry regions, particularly 
in deserts, the amount of evaporation exceeds that of the rainfall, and 
the soil is kept constantly dry, because the air is always greedy for 
more moisture than it can find. 

Dew. — During tlie summer, and at other times when 
the temjDerature does not fall below the freezing point, the 
setting of the sun is often followed by an increasing damp- 
ness of the grass and other objects that are near the ground. 
This dampness we call dew, and it may form, or "fall," 
even before the sun has finally set; or possibly its forma- 
tion may not begin until late at night, and during some 
nights no dew forms, especially if the sky is cloud}^ 
Sometimes dew gathers only in certain places, and again, 

covered with a woollen cloth. So long as the cloth is kept damp, the 
evaporation of water from the sm^face keeps the canteen and its contents 
cool, even though exposed to the direct rays of the sun. In dry countries 
one may, therefore, have cool drinking water even on the hottest day. 

1 There is a similar instrument which is not whirled, but kept station- 
ary. In this a wick leading from a dish of water keeps the muslin damp, 
the water rising as oil does through a lamp wick. 



MOISTUBM IN TBE ATMOSPMEEE 131 

particularly after a muggy sumnier day, so much gathers 
that all vegetation is dripping wet, as if with rain. 
Shortly after sunrise the glittering drops of dew disap- 
pear, being evaporated under the warming influence of 
the sun. 

The production of dew depends upon the change in 
relative humidity. The air, perhaps very humid, as is 
sometimes the case in summer, is cooled by radiation after 
the sun sinks in the west; soon the dew point is reached 
(Fig. 59), and from the then saturated air, some vapor 
passes into the form of liquid water, just as vapor in a room 
may condense on the surface of the cold window. Those 
objects that have cooled most, receive the greatest supply 
of dew, and vegetation, which is one of the best of radia- 
tors, is most abundantly supplied.^ With many clouds in 
the sty, radiation is checked, and the dew point may not 
be reached; and also if the air is very dry, the cooling at 
night may not go far enough to reach the dew point, or 
perhaps only just far enough for a little to collect. Silently 
it accumulates, not hy falling^ but by condensation from the 
air upon the surface of those objects that are coolest. 

Probably this cause for dew formation is aided by another, which 
also results from radiation. At all times vapor is being exuded from 
the damp ground, and particularly from plants, in which it rises from 
the ground in the form of sap. During the daytime this is evaporated, 
and does not appear in the form of drops of water which are visible ; 
but at night, when the air is nearly or quite saturated, evaporation 
cannot proceed, and the dampness accumulates on the surface of the 
ground and plants, adding to the quantity that comes from the air. 
This is why dew gathers on the under side of leaves and other objects 
spread out near the ground. 

1 This is one of the many beautiful adjustments of nature, by which 
animals and plants make use of Nature's riches. 



132 FIUST BOOK OF PHYSICAL GEOGRAPHY 

Frost. — This condensation of vapor from the air pro- 
duces frost whenever the temperature of the dew point is 
32° or less. Frost is not frozen dew, but merely the solid 
form assumed by the condensation of invisible vapor, at a 
temperature below the freezing point. It is quite like the 
formation of frostwork on the window, where the frost 
crystals may be seen to grow as solid forms, without any 
previous deposit of liquid water which can freeze. There- 
fore the remarks that have been made about dew apply 
quite fully to frost. 

There are many peculiarities in the distribution of frost. 
Sometimes it is very heavy, and the grass and earth are 
white with it, but at other times there is only a very light 
frost in a few places. In the latter case very slight differ- 
ences in exposure, or in the nature of the ground, will 
cause differences in amount; and indeed in one place dew 
may accumulate, while in others frost gathers. The smoke 
of a fire, or a cloth spread over a plant, will often prevent 
frost by checking radiation, and thus the cooling. Low 
ground is visited earlier than the higher land, particularly 
if the lower ground is damp ; for then there is more vapor 
in the air. It is partly for this reason that in autumn 
the leaves of trees in swamps turn earlier than those on 
the dry hillsides. There is another cause besides this one; 
for as radiation proceeds, and the air near the ground 
cools, it becomes heavy and slides down the hillsides, 
thus causing movement and a stirring of the air, while in 
the valleys the cold layers settle and remain much more 
quiet. Moving air is not easily cooled, because as soon 
as the radiation from the ground has lowered the tem- 
perature of the air near it, it slides away, and other layers 
take its place. 



MOISTUBE IN THE ATMOSPHEBE 133 

Fog. — Sometimes, particularly in damp places, the 
cooling of the ground by radiation chills the air for some 
distance above it, and lowers its temperature to the dew 
point. Then vapor must condense, and a veil of fog 
forms. In the early morning this mist may often be seen 
spreading over a swamp or a stream bottom. The fog 
particle is a minute drop of water, so small that one may 
sometimes walk in a fog without becoming sensibly wet, 
and so small also, that the particles do not settle to the 
ground by their own weight. The centre of the fog par- 
ticle is often, if not always, a speck of dust, and it is 
believed that one of the main causes for the abundant fogs 
of London is the presence of innumerable dust particles 
furnished from that great cit3^ and thus available for the 
condensation of vapor to form the fog. 

There are various ways in which fogs may be produced, 
aside from that mentioned above. When one breathes 
into the air of a frosty morning, he forms a tiny fog, 
because the warm, vapor-laden breath has its temperature 
reduced by the cold air, until the dew point is reached. 
On a very large scale nature is making fogs of a similar 
kind. In the Atlantic, along the path of the European 
steamers, near Newfoundland, extensive fog banks abound. 
Here there are two currents of water, one cold and moving 
southward from the Arctic, the other warm and flowing 
northward from the Tropics. The former is the cold Lab- 
rador current, the latter the Gulf Stream. When winds 
from the south pass over the warm Gulf Stream, and after 
becoming warm and humid pass on over the cold Labrador 
current, they are often chilled until their temperature is 
reduced to the dew point, when a fog is produced. Some- 
times a similar fog is caused on the land, when a warm, 



134 



FIRST BOOK OF PHYSICAL GEOGEAPHT 



humid wind from the south passes northward over the cold 
land, in autumn, spring, or sometimes even in winter. 
During such conditions, hov/ever, the fog may not extend 
over all the land, but occurs only over low, swampy places 
or lakes. 

On the other hand, a cold wind blowing over the warm, 
huniid earth may cause the dew point to be reached in 

the layers that 
are near the sur- 
face. Some of 
the fogs of the 
Gulf Stream 
region have the 
same origin as 
this, when cold 
air from the Lab- 
rador current 
flows over the 
Gulf Stream, 
chilling the 
warm, humid air that exists there. By one of these sev- 
eral causes fogs may fill valleys, so that from the enclosing 
hills or mountains one may look down upon a great sea 
of fog (Fig. 61), through which perchance the church 
steeples rise, while all else is hidden from sight. The 
fogs of the land soon disappear before the warm sun, 
which eats them up b}^ evaporation, as if by magic ; but 
the heavy fog banks of the ocean, or of the Arctic, may 
remain for days, until a change in the weather causes 
them to disappear. 

Haze. — Oftentimes, particularly in summer and autumn, the air is 
blue and hazy, so that distant landscapes are softened by a veil of 




Fig. 61. 

Upper surface of a sea of fog. Looking down into a 
valley from a mountain. 



MOISTURE IN THE ATMOSPHERE 135 

haze which otherwise might not be detected. Sometimes the haze so 
increases that distant objects are obscured. This phenomenon is 
generally due to the presence of an unusual number of dust particles ; 
and after dry spells, when forest fires have been extensive, and rains 
have not come to remove the dust, the air may become exceedingly 
hazy. In addition to these causes, it seems probable that some haze 
results from a form of liquefied vapoi", in which the particles are even 
less numerous and more minute than in the lightest of fogs. 

Mist. — There are times when the air is filled with a mist of par- 
ticles larger than those of fog, and yet smaller than the usual rain- 
drops. This mist may be due either to the growth of the fog particles, 
until they become so large that they settle to the earth, or else to the 
comhination of various such particles, until the same result is produced. 
The latter may happen when wind is blowing the fog about, so that 
the movement will cause numerous collisions of fog particles, until 
they grow so in size that they must sink toward the earth. Then as 
they settle, they strike other particles, and so increase in size still 
more. 

Clouds : Cloud Materials. • — A cloud may be formed of 
smoke, or of steam issuing from a locomotive; but in 
nature nearly all clouds are caused by the natural conden- 
sation of vapor in the air, when the temperature reaches 
the dew point. Therefore we may expect that clouds will 
be composed either of fog, mist, rain, snow, or ice parti- 
cles. Balloonists and travellers among high mountains 
prove that this is actually the case, for in such journeys, 
clouds are entered and even passed through. A fog or 
mist may be truly said to be nothing more than cloud 
resting on the earth. In climbing a mountain one may 
see a cloud above him, he may enter it, perhaps finding 
it to be only fog, and passing above it, and looking down 
upon its upper surface, he may see the same appearance as 
that caused by a veil of fog in a small valley. Indeed, 
during a rain storm, when the clouds rest upon the hill- 



136 FIRST BOOK OF PHYSICAL GEOGRAPHY 

sides, one may easily ascend into them and see exactly of 
what they are made. 

Forms of Clouds. — There is nothing in nature more 
beautiful than the forms and colors assumed by clouds. 

Being com- 

/ ^^^klflP ' us, "we perhaps 

^^^HHHH|^|HHB them as much 

JHH^^^^^^^^^^^^I attention as 

. ,. fl^^^^^^^^lsPf^^^P they deserve. 

^^^^^^^^^^^^^K''^ -.JH They dot the 

gdHHHHH^^^^I^HHK "^m sky in patches 

U^^^^^M/j^^^^^^^^K^^ j or 

B^^I^^^^^^^^I^^^^^L Jll ^^^^^^ every 

B^^^^^^^^^H^^^^^^HhB^H variety of 

^^^^^^^^^^^^^^^^^^^^^^HflH form, con- 

^^^^^^^^^^^^t^^^^^^^^^^^^^M stantly 

'<^^raHHH|H^^|^^^|H||H|^r ^^ would seem 
^""^^^mmmmmmmB^^^KSBS^^^^^ ^^ almost im- 
FiG. 62. 'x.^ J. ^ 

possible task 

Clouds upon a cliff in the Yosemite. 

to classiiy 
and name all clouds, and so indeed it would be, if we 
tried to find a name for every variety of form. How- 
ever, there are certain types which are fairly easy to 
recognize. 

Sometimes the sky is nearly or quite overcast by clouds, 
massed into layers or bands, giving them a stratified 
appearance. These are called stratus, and they generally 
lie low in the heavens, perhaps with their bases resting 
against the distant hillside. It is this class that accom- 



MOISTURE IN TEE ATMOSPHERE 



137 



panies the cyclonic storms described in tlie last chapter. 
Another type is that which comes on a hot summer day, 
and is commonly called the "thunder head" (Fig. 56'). 
The name for this is the cumulus (Fig. 63), and it consists 
of a bank of cloud particles rising from a nearly level base, 
whose elevation is several thousand feet above the surface. 
Above this, domes of cloud masses rise several thousand 




Fig. 63. 
Cumulus clouds. 



feet higher. These are among the most beautiful of the 
clouds, and when seen in the east after a summer thunder 
storm, especially when lighted and colored by the rays of 
the setting sun, they furnish a spectacle which may well 
arouse our admiration. From both stratus and cumulus 
clouds, rain may fall, and the rain-producing cloud is 
called the nimbus. Both stratus and cumulus clouds are 
generally so dense, that when they pass before the sun, its 
rays are cut off. 



138 



FIRST BOOK OF PHYSICAL GEOGRAPHY 




Oftentimes there are thin clouds, which scarcely or only 
partially intercept the sun's rays. These are high in the 

air, and measurements that 
have been made, show that 
they are often five or six 
miles from the earth. Some- 
times they are so thin and 
veil-like that the stars shine 
through them at night. It is 
known that these are made, 
not of fog particles, but of 
ice spicules, which are so 
transparent that light easily 
passes through. They are so 
high that the liquid form of 
water cannot exist, because of 
the low temperatures which 
occur there even in summer. These, which are often 
plumed and feathery, are called cirrus clouds (Fig. 64), 
and they are the 
highest of alL 
It is in them 
that coronas, 
halos, etc., are 
often devel- 
oped. 

Between these 
three types there 
is every gradation : 
sometimes the cir- 
rus are stratified, Fig. 65. 
and they are then Strato-cumulus clouds. 



■ Fig. 64. 
Cirrus clouds. 




MOISTURE IN THE ATMOSPHERE 



139 




Fig. m. 
Cirro-cumulus clouds. 



called cirro-stratus ; at times the feathery form is replaced by little 
banks, resembling small cumulus clouds high in the air, and these 
are called cirro-cumulus (Fig. 66), or if they are more like cumu- 
lus than cirrus, 
cumulo-cirrus. By 
combining the 
three names into 
similar compound 
words, names may 
be formed for 
most of the com- 
mon clouds of the 
sky (Fig. 65). 

Causes of 
Clouds. — Gener- 
ally the cause of 
clouds is the same 

as that of other visible forms of water in the air, — the condensation 
of vapor. The most common way in which vapor is condensed in the 
air, is by lowering the temperature to the dew point. This may 
be caused by convection, and the cumulus clouds are commonly formed 
by this means. The air near the surface rises upon being warmed, 
and as it does so, cools (Fig. 18). Starting with a certain amount of 
vapor, if the rising continues, this cooling must bring about condensa- 
tion whenever the proper temperature is reached, as it will be at a 
certain height, the elevation of which will depend largely on the 
relative humidity of the air at the beginning. This is why cumulus 
clouds have level bases, for these represent the elevation at which 
condensation began ; and as the air continues to rise, more vapor 
condenses above this, forming the beautiful piles of cloud banks. 

Vapor-laden air, coming in contact with a cool surface, may form 
clouds, exactly as it may cause fog near the ground. From this cause 
clouds often gather around mountains and even hills. Or, again, as 
in the case of fog, air currents of different temperatures may produce 
clouds. For instance, a cold layer of air moving over a warm and 
humid layer, may chill the latter near the contact and cause clouds to 
form. That there are such currents in the air, may be inferred by 
watching the clouds, when it may often be seen that those at different 



140 FIRST BOOK OF PHYSICAL GEOGRAPHY 

levels are moving in two or more directions. Probably many of the 
clouds of the upper air are caused in this way, and it now seems cer- 
tain that this is one of the causes for the dense layers of stratus clouds 
in cyclonic storms. 

Rain. — If as a result of any one of the causes mentioned 
above, the condensation of the vapor forms particles large 
.enough to fall through the air, rain is formed.^ Often- 
times such drops start from the cloud and fail to reach the 
ground, being evaporated on the way, because not enough 
drops are produced to satisfy the dryer layers of air 
through which they are passing. We may often see 
streamlets of such rain descending from the summer 
clouds and gradually dying out in the air. 

A fog particle may grow to the sizrC of a raindrop by 
condensation of vapor around it, so that its size constantly 
increases ; and then, starting to fall from the cloud, other 
particles are added to the drop by collision, until perhaps 
the raindrop has grown to large size. There is every 
gradation from these down to the tiny fog particles. 

Clouds furnish the birthplace for most raindrops, and 
their production is merely a continuation of the process 
which makes the cloud ; but a cloud may be formed with- 
out going far enough to cause rain, as we all may see from 
the fact that rain fails to fall from most of them. When 
the process of raising air by convection, or chilling it by 
contact with colder bodies, either of air, water, or land, has 
gone far enough, rain must fall, and this is particularly 
liable to happen when warm humid air is present, for then 
there is much vapor to condense. This is the case during 
the hot days which prevail in the humid tropical belt, and 
in our own country when thunder storms develop in thq 
1 Provided the temperature is above the freezing point. 



MOISTURE IN THE ATMOSPHERE 141 

hot summer afternoons. The rising air in hurricanes, 
the air currents in cyclonic storms, and the damp air of 
the ocean blowing against rising land, along the margins 
of continents, also bring about conditions favoring the 
formation of rain. 

Hail. — Rarely, in summer, when thunder storms are present, balls 
of ice fall to the ground, sometimes of such size and with such force 
as to break windows and cause much damage to vegetation. These 
are really ice balls made of layers of clear and cloudy ice, and they 
represent the freezing of water high in the air, where the temperature 
is low. Little is known of the mode of formation of these remarkable 




Fig. 67. 

Photograph of large hailstones. A ruler, marked in inches, shows the size of 

the hail. 

hailstones, but they are an unusual result of vapor condensation, and 
are apparently formed when the air is in violent commotion, and cer- 
tainly at an elevation where the temperature is low. They differ from 
snow in not being made of feathery crystals caused by the solidifica- 
tion of vapor. 

Snow. — During a winter storm, when the temperature 
is near the freezing point, a heavy, damp snow may fall, 
and reaching the ground, cover it with slush. Later, by 
a slight rise in temperature, the snow may change to rain, 



142 FIRST JBOOK 0:e' PHYSICAL GEOGRAPBT 

although there has been no difference in the appearance 
of the clonds. After the storm clears, we may see that, 
while rain has been falling on the low ground, the neigh- 







Fig. 68. 
Photograph of actual snow flakes. 

boring highlands have been whitened with snow. During 
such a condition we may leave the rain, and climbing a 
high hill, ascend into the region where snow is falling, 
passing first through the zone where rain and snow fall 
together. 

Or the storm may start as rain, and gradually, as the 
thermometer falls, change to snow, until finally the deli- 
cate crystals or snow flakes are dry, and as they fall 



MOISTTTBE IN THE ATMOSPHERE 



143 



RAIN GAUGE 



H^crrtzantcO. Secti<m,.sr. 



accumulate on the ground. These snowflakes are true 
crystals of feathery and beautiful form, and they are the 
result of the operation of a law in nature whose products 
are well known to us, but whose cause is not understood. 
Thip law is, that upon solidification from the liquid or 
vaporous condi- 
tion, many sub- 
stances will take 
on definite geo- 
metrical forms, as r 
quartz md other 
crystals cic, or as 
salt may be made 
to do by allowing 
a solution of salt 
in water to evap- 
orate. 

In the case of 
snow, the vapor is 
condensed at a 
temperature below 
freezing point, and ^^"1 g^uge^ ^'T^Z7^'fl"' ^'^"f^^^^^^^^^f ^' 

or- ' ^ a^ a (and small left-hand figures), tne funnel. 

hence one at which 

water cannot be produced, so that as vapor is given out, 
it takes the solid form directly. So the snow crystal 
gradually grows, following the definite laAvs of crystal 
groAvth, until the beautiful snow flake is formed by con- 
stant additions of vapor. There is a great variety of form 
in these flakes, but they all follow the same law of geo- 
metrical perfection. Snow crystals are not frozen rain, 
for this would form balls of ice or sleet ; but they are truly 
the result of crystallization of water vapor. 



n-c7it new. 


r.rtisca S6ctioru 


iiillillil llliillilifllli ^ 


1 


iiiilii iiJiliiHl 


V ^ A 


1 1 


) 


1 


i 


II 


3 


C 


-n 


II, 








c I 2 3 « s fi 7 e s 10 >i tz 13 14 IS IS 17 la la aoiizz23£t-i 



Fig. 69. 



144 FIRST BOOK OF PHYSICAL GFOGRAPHY 

Measurement of Rainfall. — The instrument for measuring rain- 
fall is called the rain gauge. This is a cylinder of metal which stands 
in another cylinder whose area is 10 times as great ; and upon it is 
a funnel whose top has the same area as the larger cylinder. The 
rain falls upon the funnel in the same amount as it would on the 
ground, and running down the sides, escapes through a small orifice 
which opens into the small inner cylinder, where the water collects in 
the bottom. Since the area of this inner cylinder is y^ that of the area 
of the collecting funnel, the depth of the water is 10 times as great 
as it would be if gathered in a cylinder with the same area as the 
funnel. The object of this increase is to make it possible to measure 
even small rains, for by this exaggeration of depth, an inch of rain 
becomes 10 inches deep. The measurement of tlie depth of the 
rain is made by means of a stick graduated in inches and tenths of 
inches. By an inch of rain it is meant ^hat had the rain stayed where 
it fell, it would have formed a water layer one inch deep. By various 
means the rainfall may be automatically recorded. 

For measuring the snowfall, the rain gauge may be used, the snow 
being melted, and then the water measured with a stick, as before ; or 
the depth of the snow upon a level tract may be measured. Since it 
is customary to report snowfall in its equivalent amount of rain, it is 
necessary to convert this measurement of snow depth into rainfall. 
No perfect rule can be given for this change, because the amount of 
rain represented in a fall of snow, varies with its dryness or dampness. 
There is more water in a given depth of damp snow than in an equal 
depth when it is dry. However, in ordinary snow a depth of about 
10 inches is equal to one inch of rain. 

Nature of Rainfall. — There is much difference in rain. 
Sometimes the drops are tiny, almost like fog particles, 
while at other times they are large. In some cases, espe- 
cially when the air is very humid, as in summer, the drops 
are large and very numerous, so that in a short time, per- 
haps in a quarter of an hour, an inch of rain falls, while 
in other cases, when the drops are small and not near 
together, the rainfall of several days may not make an 
inch. There is also much difference in the snow, some, 



MOISTURE 7iV THE ATMOSPHEBE 145 

especially in midwinter, being very dry and feathery, while 
in other cases, when the temperature of the air is nearly 
down to freezing point, the snow crystals are damp and 
matted together. 

Distribution of Rain. — As a general statement, it may 
be said that there is a decrease in the amount of rainfall 
from the warm tropical belt toward the poles (Plate 12). 
This would be expected, because the warm air of the 
equatorial regions carries much vapor, while that of the 
polar zone has little to give. Hence a slight change in 
the temperature of the former place will cause more vapor 
to be condensed than a great change in the colder latitudes. 
Also there is generally a heavier rainfall on the ocean, ^ 
and near the coast, than in the interior of continents. 
This again is easily understood, for the air over the 
water has more vapor than that far from the sea. In the 
United States this is very well shown, for the rainfall 
decreases from the Atlantic and Gulf coasts, westward and 
northward, until near the base of the Rocky Mountains 
there is not enough rain for purposes of agriculture. The 
same difference is shown in Russia, the steppes of south- 
eastern Russia being very much dryer than the climate 
of western Europe. 

This difference between seashore and interior is even 
better illustrated when the air that goes inland is obliged 
to pass over mountains on its way, as is the case in the 
desert and semi-desert region of the Great Basin, between 

1 The rainfall on the ocean is known to be heavy, although few meas- 
urements have been made there. Vessels move about from place to place, 
and it is only upon the islands that measurements of rain can be kept for 
any length of time. Hence the rainfall chart does not include the 
precipitation over the ocean. 



146 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the Sierra Nevada and Rocky Mountains. Here the air, 
flowing up the mountain sides, is cooled, and hence caused 
to give up much of its vapor, so that when it descends 
on the other side, and passes over the plateau, it is dry. 
This is one of the two most important causes for deserts, 
and it explains the Great American Desert of Arizona. 

Since winds which blow over mountains lose their vapor 
as the air rises, the sides of the mountain against which 
the wind blows are well watered, and particularly if the 
air comes from the sea. This is shown in desert countries, 
like our Great Basin, where mountains rise above the arid 
plateau. So little rain falls upon the lower ground that 
trees cannot exist, and other forms of vegetation grow 
only scantily (Fig. 70) ; but in the mountains rains are 
abundant, and forests exist. Even in a distance of a 
few miles there may be a great difference in rainfall. 

In the United States this cause accounts for the heavy 
rain in the state of Washington, where the prevailing 
westerlies blow from the warm Pacific water against the 
sides of the mountains. It is also shown on the world 
chart, wherever the trade winds blow from the ocean 
against the rising continents, as in Central America and 
the coast of South America (compare Plates 8 and 9 
with 12). 

The best illustration of the effect of mountains that can 
be found in the world is in India, where the warm, humid 
summer monsoon wind blows from the Indian Ocean 
against the mountains. Here is found the heaviest rain- 
fall in the world. In most of eastern United States the 
rainfall varies from 30 to 60 inches, but there it is 493 
inches. The heaviest rainfall in this region occurs during 
the months of June, July, and August, and sometimes 



MOISTURE IN THE ATMOSPHERE 147 

more rain falls in a single day than we have in a great 
part of the eastern United States in an entire year. So 
heavy is the fall of rain that all the soil is washed from 
the rocks of some of the steep mountain sides. 

When the trade winds blow over the land toivarcl a 
coast, the air is coming down grade and becoming warm; 
and hence, instead of yielding vapor, they have their 
power to take it increased. Then they are drying winds ^ 
and as a result deserts are often produced. This is the 
reason why some west coasts in the trade-wind belt are 
arid, as in the case of western South America. The 
desert of Sahara is explained in a similar way. Here 
the trades, after rising over the highlands of northern 
Africa, both descend and go to the southwards, thus hav- 
ing a double cause for warming, and hence for being 
drying winds. 

Where the air rises in the belt of calms, it cools and 
gives up vapor, forming the copious rains of that belt, 
which is one of the rainiest in the world. As the belt of 
calms migrates northward and southward with the seasons 
(Plates 8 and 9), this belt of heavy rains changes position, 
and in this way the southern edge of the Sahara region, 
though dry in one season, is well watered in the oppo- 
site. 

The rain of temperate latitudes is partly due to similar 
convectional movement, as illustrated in our thunder 
storms, partly to air coming from the ocean, and moving 
both up grade and northward toward colder regions, and 
partly to the great cyclonic storms, which pass over the 
country, and which in reality furnish us with most of 
our rainfall (Chapter YIII). In the belt of prevailing 
westerlies, west coasts are more humid than east coasts, 



148 FIRST BOOK OF PHYSICAL GEOGRAPHY 

because the general direction of the air is then from ocean 
to land. 

Distribution of Snowfall. — Over a great part of the earth snow 
never falls, though everywhere the clouds of the upper air are formed 
in a zone of perpetual cold, where vapor never condenses in any other 
than the solid form. Therefore if a mountain peak reaches high 
enough, even at the Equator the temperature may be sufficiently low 
to cause snow in place of rain. In the temperate latitudes, freezing 
temperatures are often found on the high mountains, even during the 
summer, so that in such places rain rarely falls. Upon these moun- 
tains, such as the Alps, there are great snow fields, from which 
glaciers may extend down into the valleys (Chapter XVII). 

Over the greater part of the temperate lands, some snow falls every 
winter ; but it is only in the higher latitudes that much accumulates 
on the ground. Even though situated on the same parallel, less snow 
falls in the dry interior lands than near the coast, where the air, 
though warmer, contains more vapor to furnish snow. Nevertheless 
there is less snow exactly on the coast than at a short distance inland, 
because though there is more vapor, the temperature is often so high 
that rain falls, while snow comes over the cooler inland. This is 
very well shown on the New England coast, where the snowfall is 
much less at Nantucket and Cape Cod than in central Massachusetts ; 
and in New York, where less falls in New York city than at Buffalo. 

Within the Arctic circle most of the precipitation is in the form 
of snow, though in summer, even as far as explorers have gone, some 
rain falls over the sea and on the land near sea level. On the higher 
ground of these latitudes, snow falls both in summer and winter, and 
so throughout the year this portion of the earth is wrapped in snow 
and ice, and great glaciers cover most of the land (Chapter XVII). 



CHAPTER X 

CLIMATE 

Meaning of the Word Climate. — Every day there are 
changes in temperature, and perhaps also in wind direc- 
tion, abundance of clouds, dryness or dampness of the air, 
etc. These changes from day to day are called changes in 
the iveather. Climate includes and averages these weather 
conditions. Thus we say that some places have a dry cli- 
mate, others humid, some variable, others equable. A 
variable climate is one in which the weather changes 
frequently, while in an equable climate there is little 
weather change. Therefore, to consider climate we must 
look somewhat at the question of weather. The elements 
of the w^eather are wind, rain, clouds, sunshine, tempera- 
ture changes, etc. ; and the elements of climate are the 
same, excepting that less attention is paid to the single 
cases, and more to the general result. 

Climatic Zones. — One might divide the earth into cli- 
matic zones on the basis of rainfall, or any of several features 
of air change ; but it is most common to make temperature 
the basis. A perfectly natural division may be made 
according to the altitude of the sun in the heavens, and 
therefore according to the latitude of the place. This 
gives us the tropical, temperate, and frigid zones, five in 
all ; but in any one of these there is so much difference 

149 



150 FIRST BOOK OF PHYSICAL GEOGRAPHY 

from place to place, that in each of the zones there are 
many different climates. For instance, there is a great 
difference between the climates of Florida and Boston, 
and between each of these and that of San Francisco, or 
St. Louis, or Helena, Montana, though all of these are in 
a single great zone, the north temperate. 

There are many variations in climate, and to discuss 
this subject fully would require several volumes of this 
size. However, some idea of the general climatic features 
of the world may be gained, if we take only a few places 
as types. To do this properly, it will be necessary to 
further subdivide each of the zones into oceanic (or sea- 
coast and insular), interior, and mountainous. Then also 
there are desert climates ; and within the tropics there are 
differences between the climate of the trade-wind belt and 
that of the doldrums. 

There is very much less difference in the tropical and 
frigid zones than in the temperate. In the latter there is 
an almost interminable variety, and while in each of the 
former zones there are also differences in climate, these 
are less, because the tropical zone is prevailingly warm, and 
the frigid, cold, while the places in the temperate zone 
may be now warm and now cold. Moreover, those por- 
tions of the temperate latitudes which are near the tropics, 
have much higher temperatures and less variation than 
those near the frigid zones. In the consideration of the 
climates of the world, we will first take the warm equa- 
torial belt, then the frigid zone, and then the temperate. 

Climates of the Tropical Zone : Belt of Calms. — This is 
the zone where the midday sun stands nearly vertical at 
all times of the year, and it is therefore the warmest belt. 
Here it is that the air is rising, as the trade winds blow 



CLIMATE 151 

in from either side. Calms prevail here, because the air 
ceases to move horizontally, and ascends. In this belt 
sailing vessels may linger for days, and even weeks, with- 
out having a wind of sufficient force to drive them through 
to the zone of the trade winds. On the ocean, where the 
doldrums are best developed, the air both of day and night 
is warm and humid throughout the year. The heat of the 
daytime, although great and oppressive, is tempered some- 
what by the ocean, and this is the most equable climate 
in the world. Because the air is rising, and hence cool- 
ing, there is heavy rain throughout the year ; and during 
the day, when the sun shines, and the air rises more per- 
ceptibly, clouds form and copious rain falls. 

Over the land there is less rain and the temperature 
is less equable. , Because of radiation from the land, 
the daytime is hot and the nights cool. Because of the 
absence of ocean water to supply vapor, the air is less 
humid, and hence the rainfall is less. Also the air is less 
calm, for over various parts of the land there are differ- 
ences in temperature, which cause breezes to arise. Never- 
theless, even over the land, this is a rainy, relatively calm, 
and very warm belt. In it are situated the heavily forested 
rainy districts of central Africa and the Amazon valley. 
These change gradually to less dense forests on either 
side, and in Africa gradually give place to the dry desert 
of Sahara, on which almost no vegetation can grow. 

As the belt of calms migrates with the seasons, the position of the 
heavy rains changes, so that on the margin of the tropical rain belt, 
there is a climate which is dry in one season and very wet in the 
other. This is shown in South America, where the llanos of Vene- 
zuela are rainy during the summer season, and dry in the winter, and 
also in the campos of Brazil, south of the Equator, which are well 
yv^atered in winter and dry in summer. 



152 FIRST BOOK OF PHYSICAL GEOGRAPHY 

The Trade-Wind Belt. — There is much more variety in 
the climate of this zone than in the belt of calms. Being 
within the tropics, the temperature is everywhere high 
excepting on the lofty mountain tops, where the climate 
is almost frigid. Among these mountains one may jour- 
ney from the zone of perpetual summer, to that where the 
conditions of spring prevail throughout the year, and 
then, going still higher, one may rise above the elevation 
where timber can grow, and finally into a region where 
the nights are cold and the days cool, and perhaps even 
as 6old as those of the northern winter. 

Although in respect to temperature there is a resemblance between 
these climates of high altitudes and those of the frigid zone, there is 
this difference, that even though the weather is cold, the sun rises 
high in the heavens in midday at all times of the year. 

Over the ocean the trade winds blow with wonderful 
steadiness, moving constantly, and with distinct strength, in 
one direction. They are warm and carry much vapor, taken 
as they pass over the sea; but since they are blowing from 
colder to warmer latitudes, their temperature is constantly 
rising, and hence they are able to take much more vapor. 
Therefore as they blow over the sea, they do much work of 
evaporation, and here they are not especially rainy winds ; 
but that the trade winds are carrying much vapor is proved 
by the fact that when the air has its temperature lowered, 
when rising in the belt of calms, it precipitates quantities 
of moisture. 

When blowing over the land, the trade winds may pro- 
duce deserts, as they do in the Sahara north of the Equator, 
and in Australia and South Africa south of the Equator. 
The desert climate is one of extreme heat in the day, fol- 
lowed by a cool, or in winter really cold, night (Fig. 25), 



CLIMATE 



153 



Because of the dryness of the air, which permits heat to 
readily reach the ground throughout tiie daytime, and 
allows it to be radiated at night with almost equal ease, 
the temperature range of the desert is great. As the name 
indicates, a desert climate is one of great dryness, rarely 




Fig. 70. 
The desert vegetation in tlie far west. 

if ever a climate in which no rain falls (Plate 12), but one 
in which there is little precipitation, and this only in cer- 
tain seasons. In the desert there is not enough rainfall 
to support any but the most hardy forms of desert plants. 
On the land, the direction of the trade winds is often 
changed as one part of the land becomes warmer than 
another, causing a circulation as the air attempts to 
equalize the differences in pressure thus produced. In 



154 FIRST BOOK OF PHYSICAL GEOGBAPBY 

this way the trade winds are sometimes deflected from their 
course, and caused to move toward the heated land, as in 
the case of western Africa (Plates 8 and 9), and also on 
many oceanic islands, where the sea breeze blows and air is 
drawn in from the ocean. Then the climate of the trade- 
wind belt may be changed from one of moderate dryness 
to one of heavy rainfall (Plate 12); for as the air blows 
in over the heated land, either passing up the grade of 
the land, or rising by convection, the dew point is soon 
reached, clouds are formed, and rain falls. The trade- 
wilid belt is also rainy on many east-facing coasts, for 
as the air blows against the land, being forced to rise, it 
gives up some of its vapor, causing heavy rains, as in the 
case of South America both to the north and south of the 
Equator (Plate 12). 

The Indian Climate. — A peculiar climate is found on the plains of 
India, within the trade-wind belt, where the air movement is modified 
by the monsoon effect of Asia. Here there are three seasons, the hot 
summer, the rains, and the winter. The hot summer begins in April 
and lasts until June, and during this time the air is hot and dry, and 
the temperature of the day reaches above 100° in the shade, and 
sometimes 110° or 115°. Everything becomes dry, and it is almost 
impossible for an Englishman to take exercise, excepting at night and 
just before the dawn. Everything withers before the scorching west 
winds which blow from the sandy wastes of the Indus valley. 

Then in June there comes a calm, in which the heat, still intense, 
becomes even more suffocating, because there is no movement of the 
air ; and every one prays for the south and east winds of the summer 
monsoon, which bring rain and some relief from the steady heat. 
Finally clouds appear, and rain falls, which during this season, last- 
ing over a month or two, is of daily occurrence. Under the influence 
of the heavy rainfall, plants flower a second time, having previously 
been in leaf and flower in February or March. The intense dryness 
is followed by equally intense dampness, and then, toward the close 
of the rains, the climate is once more almost unendurable. 



CLIMATS 155 

By the beginning of October the winter monsoon begins, and from 
then until December the cool air, blowing from the highlands of 
northern and central India, transforms the hot plains to a region with 
a deliciously cool climate, in which the air is clear and dry. Then 
the weather becomes so cold that fires are needed in the evening, 
during the months of December and January. In February the warm 
weather begins, and a sort of spring visits the land, inducing vegeta- 
tion to break forth ; and this is then followed by the hot, dry season, 
which discourages the thriving vegetation, and causes it to wither 
until it again bursts forth in the ti-ue growing season of wet weather. 

Climates of the Frigid Zones : The South Frigid Zone. — 
Very little is known about the climates of the south frigid 
belt, but there seem to be two different climates, that of 
the ocean, and that of the high, ice-covered land of the 
Antarctic continent, which appears to entirely enwrap the 
South Pole. Probably these climates are very similar to 
those of tlie Arctic Ocean, on the one hand, and the ice- 
covered land of Greenland on the other; but they have 
not been studied. 

Wear the Arctic Circle. — Within the Arctic there is a 
progressive increase in the severity of the climate as one 
proceeds northward. In the extreme southern portion, 
near the sea level, the summer sun reaches about as high 
in the heavens as it does in northern United States during 
the late autumn. At night-time it drops down to the 
northern horizon, and at midnight the eartli is lighted 
either by the dim sunlight or bright twilight. Therefore, 
although the sun is low in the heavens, it shines most or all 
of the day, and the summer weather is cool, but not cold. 
The storms of temperate latitudes affect this region, and 
hence there is much variety of weather, with alternately 
cool and clear conditions, followed by a cloudy sky, per- 
haps with rainy weather, similar to the changes in the 



156 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



United States. The temperature is so high that rain falls 
instead of snow, which is the form of precipitation during 
most of the year. 

In the winter the sun rises high enough to cause twi- 
light at midday, and the night is therefore constant. 
Then there is a season of prevailing cold. The storms 
cause cold or warm winds, and clear weather or snow, and 
these changes are determined, not by the direct influence 
of the sun, but by outside causes. There is little or no 
daily rise and fall of temperature, but the variations are 

mainly gov- 
erned by move- 
ments of the 
air caused by 
the cyclon- 
ic storms of 
the temperate 
zone. Between 
this season of 
winter cold 
and summer 
coolness, there 
are two seasons when the sun rises and sets; and then, in 
addition to the causes mentioned, the temperature changes 
from day to night. There are therefore four different 
seasons, with quite distinct characteristics. 

In the Higher Latitudes. — Further north these climates 
become even still different. The winter night is marked 
by intense cold, with temperatures generally ranging be- 
tween 21° and 60° (and probably more) below zero, though 
now and then rising above freezing point, when a warm 
wind is caused by some movement of the air. During 




Fig. 71. 
The midnight sun, northern Norway. 



CLIMATE 



157 



this season the land is deeply snow-covered, and the sea 
coated with ice. As the darkness of the winter night is 
replaced by the spring-time, soon the midday becomes 
comfortably warm, while the nights are cold. Then the 
snow begins to melt, and the sea ice to break up and float 
away to the south (Fig. 72), where it eventually melts in 
the warmer waters of the more temperate latitudes. 




Fig. 72. 

The ice-covered sea off Cumberland Sound, Baffin Land, summer of 1896. 

Steamer Hope in the ice. 

After this comes the summer, when plants burst forth 
into blossom, and the hum of insects is heard, being 
warmed into life under the influence of the summer sun, 
that shines by night as well as by day. Though low in 
the heavens, the sun warms the earth, the frost disappears 
from the surface, and even at midnight the temperature 



158 FIBST JBOOK OF PHYSICAL GJEOGUAPHY 

does not descend to the freezing point. During this 
season rain may fall even as far north as man has gone. 
After this comes the autumn, when the darkness of night 
again appears, and the earth, warmed slightly by day, 
cools at night, so that the bays begin to freeze, snow com- 




FiG. 73. 
A part of the high coast of Greenland, summer of 1896. Latitude 74° 15'. 

mences to fall, and gradually the day becomes shorter, 
until finally the winter night sets in, and for weeks, and 
further north even for months, the sun is not seen. 

The same changes of sun occur in the high interior lands; but 
here, because of the greater elevation, the temperature is so low that 
even in summer there is never rain, and never warmth enough to melt 
the snow. Here perpetual winter prevails, and the land is deeply 



CLIMATE 



159 



covered with snow and ice, as in the case of the whole interior of 
Greenland, and probably also of the great Antarctic continent. This 
climate, which is bitter cold in summer, must become intensely severe 
in winter ; but no one has ever lived there to tell us how low the tem- 
peratures descend. We do know that the snowfall is extremely heavy. 




Fig. 74. 
The Greenland ice sheet showing a part of the Cornell Glacier. Latitude 74° 15'. 
Taken from an elevation of 1400 feet. Fjord in foreground 3 miles wide, and 
icebergs in it, in some cases, 75 feet high. The glacier covers all land except- 
ing the island in the ice (nunatak), which is 9 miles distant. 

There are other differences in the frigid climate, the 
most noteworthy of which is that caused by ocean currents. 
The coast of Greenland is distinctly warmer than that of 
Baffin Land in the same latitude, because a warm current 
of water, coming from the south, bathes the former shores, 
while a cold current, flowing southward, passes Baffin 
Land and carries to that coast the chill of the ice-laden 
waters of the north. 



160 



FIB ST BOOK OF PHYSICAL GEOGBAPHT 



Climates of the Temperate Zone : Various Types. — Near 
the frigid zones the temperate climate is quite like that 
near the Arctic and Antarctic circles ; and near the tropics 
the conditions resemble those described for the trade-wind 
belt. Between these two extremes are the great temperate 
belts of variable climates, in which so much of the civil- 
ized world lies. These two zones, one south and one 

north of the Equator, 

are characterized by pre- 
vailing moderate tem- 
perature, changing from 
warm and perhaps hot 
in summer, to cool or 
cold conditions in win- 
ter. Everywhere there 
is generally a range of 
temperature from day to 
night, and this differs in 
winter and summer, and 
also in the two interme- 
diate seasons of autumn 
and spring. These daily 
temperature changes are 
liable to be interrupted 
by the cyclonic and anticyclonic disturbances which affect 
the temperate latitudes. The wind and weather changes 
are influenced by the prevailing westerlies, and mainly 
caused either by differences in heat effect, or by the low- 
and high-pressure areas, which pass from west to east over 
the higher latitudes of the belt (Chapter VIII). 

The climates and weather of the south temperate zone resemble 
those of the northern, excepting in the fact that there is less irregu- 




FiG. 75. 

A cold wave, March 13, 1888. Temperature 
indicated by shading. Isobars also shov/n. 



CLIMATE 



161 



larity, because the southern hemisphere is occupied mainly by water. 
This allows less range of temperature, and less irregularity of winds, 
which prevailingly blow from the west. Near the tropics, in both 
hemispheres, the climate of the temperate zones is modified by the 
conditions of the horse latitudes, where the air is settling, the winds 
light and variable, with frequent calms, and the sky generally clear. 
In this belt of settling air there are some deserts, as for instance on 
the east coast of southern South America. 




Fig. 76. 
Map showing snowfall of United States in inches. All in temperate belt. 



United States Climates. — In northern United States, 
southern Canada, and Europe, we find the characteristic 
weather conditions and variable climates of the temperate 
latitudes. This variability has been sufficiently described 
for the United States in the chapter on Storms, and what 
is said there applies to Canada and Europe, and in gen- 
eral to other parts of the middle temperate belt. By the 
passage of cyclonic storms and anticyclones, the daily 
range of temperature in Avinter may be replaced by warm 
spells, or by cold weather, and in summer by hot or by 
cool spells. A cold wave, overspreading the land, may 



162 FIRST BOOK OF PHYSICAL GEOGRAPHY 

cause the temperature to drop even at midday, and cover 
the northern United States with a blanket of cold air, 
producing temperatures varying from freezing to 40° 
below zero (Fig. 75). Or on the contrary, warm winds 
from the south, moving toward a storm centre, may cause 
the temperature to rise, even at night, and to become so 
high that a thaw occurs, causing the snow to melt from 
the ground (Figs. 53 and 54). 

As these changes occur, the direction of the wind varies, 
being now from the south, now from the west, north, or 
east; and now the sky is clouded, and perhaps rain or 
snow is falling, and then the sky is clear, and the vault 
of the heavens a beautiful blue. Day by day, and week 
by week, these changes occur, and no one can tell what 
the weather will be from week to week, excepting to know 
that it will be variable, day after day. This is in striking 
contrast to the ever dry climate of a desert, the constant 
winter cold of parts of the Arctic, the uniform humidity 
of the doldrum belt, or the permanent winds of the lands 
influenced by the trades. 

Difference betiveen United States and Europe. — Within this belt 
of variable temperate climate there is much variety from place to 
place. The great city of St. Petersburg is situated in nearly the same 
latitude as southern Greenland and northern Labrador, — places in- 
habited only by Esquimaux and a few Europeans who live there for 
purposes of trade ; Berlin and London lie in the latitude of southern 
Labrador, a sparsely settled region, having a climate of almost Arctic 
rigor ; and New York lies in the latitude of southern Italy and Greece, 
places with warm and almost subtropical climates. 

There are two reasons for these conditions, one the fact that the 
prevailing westerlies blow over eastern America, after having passed 
across the land, while those of Europe have blown across the ocean 
water. The second reason is that the water of the eastern Atlantic, 
on the European coast, is warmed by an ocean current from the south 



CLIMA TE 163 

(the Gulf Stream), while the American shores are bathed by a frigid 
current (the Labrador) from the icy Arctic sea (Chap. XTII) . For simi- 
lar reasons the western coast of the United States is warmer than the 
eastern, and also warmer than the eastern coast of Asia in the same 
latitude ; but the difference between the climates of the Asiatic and 
American coasts is less than that just described, because no cold Arctic 
current bathes the shores of eastern Asia. 

Variation ivith Altitude. — There are also noticeable dif- 
ferences in the climate of the temperate zone according to 
altitude. In a small way this may be seen in any mountain- 
ous district, like that of New England, where the valleys 
are very much warmer than the mountain tops (Fig. 31). 
For instance, Mt. Washington in New Hampshire is en- 
wrapped in cold air even in summer, while the lowlands 
to the east, in New Hampshire and Maine, are covered 
with a blanket of hot air; and by autumn the top of this 
mountain is covered with snow, while in winter the tem- 
peratures are exceedingly low and the snowfall heavy. In 
the same way the Alps, which lie in the latitude of south- 
ern France, where the summer is hot and the winter not 
extreme, are so cold that snow falls upon their summits 
even in summer, so that there are great fields of perpetual 
snow and glaciers among the mountains (Chap. XVII). 

Differences between Ocean and Land. — Again there is a 
variation in climate from the sea to the interior. The cli- 
mate of New York city is warmer and more equable than 
that of Ohio, and this in turn is less extreme than that 
of Wyoming, all of which are in the same latitude. The 
climate of Boston is less severe than that of the interior 
of Massachusetts, and from place to place along the coast 
one finds many variations (Figs. 24 and 30). 

At Cape Ann, Mass., a point nearly surrounded by the equable 
ocean, the temperature during the cold midwinter does not descend 



164 FIBST BOOK OF PHYSICAL GEOGBAPHY 

so far as at places 10 miles inland; and in spring, being surrounded 
by the cold ocean water, vegetation does not so quickly develop as it 
does at Cambridge, which lies but a short distance inland. The 
leaves begin to appear upon the maple trees in Cambridge fully a 
week earlier than at Cape Ann. Throughout the fall, the ocean water, 
warmed during the summer, prevents radiation on the land from 
cooling the air over this cape to such low temperatures as those 
reached a short distance away ; and hence some frosts, which occur 
a few miles from the shore, do not visit the cape. 

These same differences in climate are shown near lakes, 
as for instance along the shores of Lake Erie, in western 
New York. Here the conditions are so equable, near the 
lake shore, that an extensive grape-raising industry has 
developed, while upon the hillsides, two or three miles 
away, this industry is not possible. 

On an even more notable scale the influence of the 
ocean upon climate may be illustrated by contrasting the 
Bermuda Islands with central Georgia on the same paral- 
lel of latitude. The former, surrounded entirely by warm 
ocean waters, does not have extremely high temperatures 
in summer, while in winter the nights are never cold and 
rarely cool, and frosts are practically unknown. The 
vegetation has a tropical aspect (Chap. XI), and the Ber- 
mudas in this respect resemble the Bahamas and Florida, 
which lie much further south. In central Georgia, the 
sun, having the same altitude, warms the land in summer, 
causing hot days, while in winter, although the climate 
is not extreme, frosts are by no means uncommon, and 
the nights are often cold. Thus in the temperate lati- 
tude there is an infinite variety in the weather; and 
equally great is the variation in the climates of different 
parts of the great zone. 



CHAPTER XI 

DISTRIBUTION OF ANIMALS AND PLANTS 

Zones of Life. — There are three great zones of life 
inhabited by different assemblages of animals and plants, 
— the ocean, the land, and the fresh water. In each of 
these there are subzones in which the animals and plants 
differ because of variations in temperature. For instance 
there are very different organisms in the tropical belt 
from those of the frigid or even the temperate zone, and 
this applies to ocean and fresh-water as well as to land 
animals. Besides these there are other zones in the sea 
and deeper lakes, for the nature of the flora and fauna 
varies with depth. Also in the larger bodies of water there 
are changes with distance from shore, and also along the 
shore, as the nature of the coast varies. There are also 
numerous subdivisions of the zones of land life. The 
creatures that live among the mountains differ from those 
of the plain, while those of the humid seacoast climate 
bear very little resemblance to those of the desert. The 
subject of the distribution of animals and plants is there- 
fore a very complex one, which in a book of this scope 
can be treated only in a brief and most general way. 

Life in the Oceai^ 

Plants. — In the sea both plants and animals exist in 
great abundance, though the latter greatly exceed the 

165 



166 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



former in importance. Excepting at the very coast line, 
none of the higher flowering plants, so common on the land, 
are found. Upon salt marshes (Chap. XVIII), there are 
numerous species of plants, resembling those of the land. 
On the coast of Florida and other tropical lands, the 
mangrove tree is able to live with its roots in salt water ; 

and in protected places along 
these coasts there are veri- 
table jungles of mangrove 
swamps (Fig. 77), into which 
the salt water enters. 

Elsewhere in the sea the 
plant life belongs to the lower 
forms of vegetation, notably 
the seaweed. Upon that part 
of the rocky coast of New 
England which is exposed at 
low tide, these plants produce 
a mat (Fig. 78), and the sea- 
weed also grows upon the 
bottom, near the coast. It is 
limited to shallow water, be- 
cause at depths of a few hun- 
dred feet not enough sunlight passes through the ocean 
water to perform the work necessary for plant growth. 
Therefore, the great expanse of ocean bottom, where the 
water is deep, is devoid of vegetable life, being in this 
respect a great desert extending over nearly three-fourths 
of the earth's surface. Seaweed needs to have a solid base 
on which to grow, and hence, on the exposed, sandy shores, 
where the waves keep the sand particles in constant move- 
ment, these delicate organisms cannot exist. 




Fig. 77. 
A mangrove swamp, Bermuda. 



DISTRIBUTION OF ANIMALS AND PLANTS 167 

There is also much seaweed floating aboat on the surface of the 
ocean, especially in the warm waters of the tropical zone in the mid- 
ocean ; and sometimes this gathers over such great areas, that sailing 
vessels have their progress retarded in passing through them. These 
are called "grassy" or Sargasso seas, like that which lies between 
Spain and the West Indies, in which the species of Sargassum are 
most abundant (Plate 16). 



Animals. — The abundance of animal life in the sea is 
marvellous, and there is no part of its surface or bed which 
is not inhabited. We may 
recognize three great zones of 
animal life in the ocean: 
(1) The littoral, or that of 
the seacoast; (2) the abyssal, 
or that of the ocean bottom; 
and (3) the pelagic, or that 
of the surface. 

Faunas of the Coast Line 
(^Littoral faunas^ . — The sea- 
coast faunas vary greatly from 
place to place. Some creatures 
swim in the surf, some cling 
to the seaweed, many attach 
themselves firmly to the rocks, 
others burrow in the mud, 
and in all of these places 
many move about from place 
to place, walking or crawling 
over the bottom. Hence as 

we pass along the coast, going from a rocky headland to 
a sandy beach, and then to a muddy flat in an enclosed 
bay, we find thre6 entirely different types of animal 




Fig. 78. 

Seaweed mat. Shore of Cape Ann, 
Mass. Exposed between tides. 



168 FIRST BOOK OF PHYSICAL GEOGRAPHY 

colonies, even though the distance be no more than a 
mile. 

There are also great differences in the animal life ac- 
cording to the temperature of the water. Within the 
Arctic almost no organisms live on the rocky shores, 
because in winter these are ice-bound, and as the tide rises, 
it grinds against the shore with such power that no life 
can withstand its effects. But below the reach of the ice, 
many animals and seaweeds exist. 

These, as well as those of the temperate latitudes, though abun- 
dant, are much less so than the myriads of creatures that dwell in the 
warm waters of the tropics. Moreover, they are much less delicate 
and beautiful ; and in their plain and rugged forms they tell of the 
struggle against the rigors of winter, to which the tropical animals 
are not subjected. The latter, bathed constantly in warm water, and 
at all times furnished with an abundance of food, become marvellously 
beautiful and varied in form and color. Similar differences are 
noticed on the land, both in the animal and vegetable kingdoms. 

It is within the waters warmed by the tropical sun, and 
particularly where this water is in circulation as warm 
ocean currents, that we find the littoral faunas in all their 
luxuriance of development. These currents not only bear 
the equable conditions of constant warmth, but also an 
abundance of floating animals, which thrive in the warm 
water. The dwellers on the seacoast and shallow bottoms, 
usually anchored firmly in place, cannot go to seek their 
food, but must have it brought to them ; and therefore 
warm currents, laden with animalculse, furnish them with 
an abundant food supply. 

Under these favorable conditions, reefs of coral develop, and one who 
has never seen a coral reef (Fig. 79), can form no real conception of 
the vast numbers and wonderful variety of the animal life clustered 



DISTRIBUTION OF ANIMALS AND PLANTS 



169 



together in these colonies. Drifting about in a boat, one may gaze 
down upon a bottom covered with corals of all colors and forms, with 
millions of mouths wide open and hungry for food that is floating 
past. The coral reefs are the gardens of the sea, and I know of no 
better comparison than to a 
garden on the land, in which 
plants of various colors and 
forms are growing in rank 
profusion. Nowhere else in 
the world are there so many 
individuals and species of 
animals clustered together 
in a small space, as one may 
see in the coral reefs of Ber- 
muda, the Bahamas, and 
many of the coasts of the 
tropics. 

Animals of the Ocean 
Bottom(^AhyssalFanna). 
— It was once thought 
that no animals could 
exist on the bed of the 

sea, where the water has a depth of at least a mile, and 
often two or three miles, and where the temperature is 
always nearly at the freezing point, and where a darkness 
like that of night constantly reigns. Now, however, as a 
result of much study, we know that this great realm is 
inhabited by animals not greatly different from those of 
the ocean surface. Fishes swim about, shellfish crawl 
over or burrow into the mud, and shrimp, sea-anemones, 
and many other kinds of ocean animals exist there in great 
numbers. Some are blind, but others have eyes. Most 
of the animals dwelling in this zone depend for their 
food supply upon the remains of animals that lived near 




Fig. 79. 

A part of the Great Barrier Reef, Australia, 
showing profusion of coral life. 



170 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



the surface of the sea, and upon dying, settled to the 
bottom. The abundance of animal life on the ocean bed 
is in large part determined by the amount of food that is 
thus supplied. 

There are fewer differences among the animals of differ- 
ent parts of the ocean bottom, than in any other great area ; 




Fig. 
A deep-sea fish. 

the temperature is always low, there are no changes with 
season, and none from day to night, and the temperature 
is about the same at the Equator as at the Arctic circle, 
being nearly everywhere below 40°, day after day, and year 
after year. There is therefore little cause for differences. 
Nearly everywhere, as the depth of the sea increases, the 
temperature becomes lower (Fig. 93), and this is one great 
cause for the variation in the faunas of the deep sea. Some- 
times warm currents of water bathe shallow parts of the 
sea bed, and then, under the more favorable conditions, 
greater numbers and different kinds of animals live in the 
shallows, than in the neighboring deeper and colder water. 



DISTRIBUTION OF ANIMALS AND PLANTS 171 



Life at the Surface (^Pelagic Faunas), — Here, there is 
great variety and abundance of animal life. Not merely 
do the larger fishes swim 
about singly and in great 
"schools" or "shoals," 
containing tens of thou- 
sands in a single group, 
but the water teems with 
life of minute and even 
microscopic size. Count- 
less myriads of these 
tiny creatures occupy 
the water of all parts of 
the ocean surface, trom the 
tropical zone to the ever 
frozen waters of the Arc- 
tic. One might sail over 
the ocean without being 
aware of their existence, 
so small are they; but if 
he will drag the surface 
with a net having minute 

meshes, he may gather these animals, and in a dish of sea 
water examine them at leisure. 

Now and then, for some unknown reason, these tiny creatures com- 
bine to produce a phosphorescent glow on the water surface, and then 
at night, a gleam of silvery light marks the track of the vessel, or sil- 
very drops fall from the oars. Each animalcule is emitting his share 
of this strange light. The abundance of these creatures is shown by 
the fact that the mammoth whale obtains his living from them, 
swimming through the water with his mouth wide open, and strain- 
ing the minute animals from the water by means of the fringed 
whalebones. These monsters obtain food by this means not only in 





^fcr' 


fi^^M 


^K 






p) 


V? 


w 




(bN 


1 


i 




^ 



Fig. 81. 
A deep-sea crinoid. 



172 FIBST BOOK OF PHYSICAL GEOGBAPHT 

the warm waters of the tropics, but even in the ice-strewn Arctic 



seas. 



There is little reason for difference in the pelagic fauna, 
excepting in places so far apart, and so different, as the 
cold waters of the frigid zone and the warm equatorial 
ocean. Within the tropics, the waters are always so warm, 
and the conditions so uniform, that the surface animals 
differ but little; they swim about easily from place to 
place, or are driven here and there before the winds or 
the currents, and hence are widely distributed. So also 
in the frigid zone, the constant cold which prevails in 
these waters favors uniformity of life. In the temperate 
latitudes, however, there are somewhat greater differences, 
and hence greater variety. Near the coast, the water is 
cold in winter and warm in summer, while further out to 
sea, the temperature is more equable, and generally higher, 
because of the presence of warm ocean currents. There- 
fore there are zones of life here. 

In the ocean there is therefore the greatest variety of 
life conditions in the littoral or seashore zone, and least 
in the great expanse of the deep sea. In each of these 
zones there is wide distribution, partly because the waters 
are in nearly constant movement, partly because many of 
the creatures can swim about, ^ and partly because the 
temperature variation is not great, excepting in widely 
separated regions. In the entire area of these three great 
zones, there is a wonderful variety and abundance of 
animals. 

1 Even those which are anchored have a free-swimming stage early in 
life before settling down to the real condition of maturity. 



DISTRIBUTION OF ANIMALS AND PLANTS 173 



Life in Fresh Water 

Some of the animals that dwell in fresh water come from the air 
and land,i but there are many species of plants and animals which 
live and die in the fresh water. As the animals and plants of the land 
vary, so do those of the lakes and rivers, for among these there are 
differences from lowland to mountain, and from frigid to tropica] 
zones. Many of the groups of animals of the sea inhabit the lakes ; 
but many, like corals, are never found in fresh water. Some sea fish 
(such as salmon, alewives, etc.) have a habit of passing up rivers into 
lakes to breed or " spawn," and hence there is often a difference between 
the faunas of fresh-water bodies near the sea and those remote from 
it ; for not only do the adults pass up stream to lay their eggs, and 
then come back again, but the young remain in the lakes for a season. 
Through changes of land and water, animals of the sea have some- 
times been obliged to remain permanently in the fresh water, and 
then sea fish are found in the lakes at all times. Probably many of 
the lake and river fish have come to inhabit their present homes in a 
similar way. 

In the small lakes, and in rivers, there are only slight differences in 
the nature of the animal and plant inhabitants from one part to 
another. There will perhaps be different creatures on the swampy 
shores from those of the rocky headlands, and between these and the 
inhabitants of the marshy and sandy shores ; but these variations 
would be slight, because the distance and the difference in conditions 
are not great. But in large lakes, like the Great Lakes, the fauna and 
flora vary greatly, and in a way similar to the conditions existing in 
the ocean, although in lakes there is no such abundant variety of 
animal life as in the sea. There is here a variable littoral fauna and 
flora, a pelagic zone, and a lake-bottom zone. In the latter, as in the 
sea, there is little variety, because the temperature is always low, and 
the conditions constant and unfavorable to life. Here also, as in the 
sea, plant life ceases below the depth where the sun's rays cease to be 
powerful enough to perform the work needed by plants. Although 
the species are not the same, and notwithstanding many minor dif- 

1 Such as the young of the mosquito and dragon-fly from the air, and 
the tree-toad from the land. 



174 



FIRST nOOK OF PHYStCAL GEOGRAPBT 



ferences, there is a general resemblance between the life zones of large 
lakes and the sea. 

Sometimes a fresh-water lake has its source cut off, as the result 
of a change in climate from moist to arid, when the water evapo- 
rates faster than the rain can supply it; and then gradually a 
salt lake results, as in the case of the Great Salt Lake and the 
Dead Sea. As the fresh water changes to salt, many of the animals 

perish, until final- 
ly only a few re- 
main, so few, in 
fact, that the lake 
is called a dead 
sea. Even in the 
Great Salt Lake 
there are some mi- 
nute animals and 
plants. The life 
of the salt lakes 
is different from 
that in any other 
part of the world, 
the chief differ- 
ence being in the 
very limited num- 
ber of species 
and individuals, 
which contrasts so distinctly with the abundance of life in ocean and 
fresh water. 

Life on the Land 

Plants. — The most characteristic life on the land is the 
plant life, and yet animals are of very high importance. 
Vegetation clothes the surface almost everywhere, furnish- 
ing dwelling-places for many animals and supplying all 
with food. Animals of the land have not the power of 
taking food directly from the earth ; but plants are able to 
convert earthy materials into substance which animals can 




Fio. 82. 
A view in a semi-tropical forest in Florida. 



mSTBlBUTION OF ANIMALS AND PLANTS 175 



use; and even those creatures which do not take their 
food directly from plants, do so more indirectly from other 
animals. Most plants live at the surface of the earth and 
cling to it, firmly anchored in place. 

There is a limit to the abundance of plants in any given 
place, but this varies with the conditions. Under the 

most favorable circum- 

stances, as for instance 
Avithin the tropical belt of 
heavy rains, the limit is only 
that of space. As many as 
can get together and obtain 
food from the earth, and the 
necessary sunlight for plant 
growth, can exist in this 
place; and here are found 
tropical jungles of forest 
trees reaching high in the 
air, and a tangle of under- 
growth, forming an almost 
impassable mat of vegeta- 
tion. 

On the other extreme, in 
deserts, even though within 
the tropics, the conditions 
of sunlight and warmth are 

still present, but the equally necessary water is absent, 
and hence the surface is only scantily clothed with the 
desert grass, cactus, and other kinds of vegetation which 
can thrive amidst such adverse conditions (Figs. 70 and 83). 

Growing easily, because of the favorable conditions, plants in the 
humid climates suffer little if attacked by animals ; but on the desert 



blIi'j >^^^SBi^l 


L 



Fig. 83. 

In the desert of Arizona, showing giant 
cactus. 



176 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



the conditions of nature are so adverse that any additional difficulties 
would be fatal. Hence the desert plants attempt to protect themselves 
from the attacks of animals, attaining this end sometimes by means of 
thorns, as in the case of cactus, and sometimes by developing sub- 
stances in the sap which cause the taste of the plant to be disagreeable, 
as in the sage brush of the desert. 

Equally adverse are the conditions on high mountain 
tops (Fig. 84), where the temperature is low, or in the 




Fig. 84. 
A mountain peak on the crest of the Andes in Peru, above the timber line. 

Arctic, where not only is there cold, but also a limited 
amount of sunshine. Approaching either of these re- 
gions, we find the trees changing from the deciduous to 
the evergreens, the forests becoming less dense, and then 
the trees more and more scattered and stunted, until finally 
the timber line is passed (Fig. 85), and the only forms of 
vegetation are those that cling to the earth. Within the 
Arctic regions the willows and other species of trees creep 



DISTRIBUTION OF ANIMALS AND PLANTS 177 

along tlie ground, raising their leaves and flowers no higher 
than is necessary; for early in the winter it is vitally 
important to secure a covering of snow which shall pro- 
tect them from the intense cold. However, no matter 
how far north we may go, even to the highest latitude 
so far visited, grass and 
j9owers will be found in 
summer, wherever soil 
exists in a position ex- 
posed to the sun (Fig. 
86). Upon the bare 
rocks are innumerable 
lichens and mosses, and 
on the hillsides, and in 
the valley bottoms, are 
patches covered with 
flowering plants, but 
there are never any trees. 
With change in lati- 
tude there are many va- 
riations in vegetation; 
the tropical plants differ 
radically from those of 

the temperate zone, being much more abundant, varied, 
and beautiful. Fruits abound, and Nature is very prod- 
igal in her productions. 




Fig. 85. 

A view near the timber line in the high 
Rockies of Colorado. 



Many of the plants of the world are cultivated by man, and even 
where he protects them, there are distinct belts in which various 
species grow best. For instance, coffee, bananas, pineapples, etc., can- 
not be grown in northern United States ; and barley or wheat thrive 
better in the cooler climates. But in the cold Arctic zone it is 
impossible to raise even the hardiest of crops, for the summer season 

N 



1T8 



mitST BOOK OF PHYSICAL GEOGRAPHY 



is too short for any but the native species, which are accustomed to 
blossom and mature their seed in the short summer of these far 
northern lands. 

Animals. — The animal life of the land may be said to 
live in three great zones, or else to spend a part of their 
time in two or all three. There are many that dwell in 
the air, and this includes many of the insects and most of 

the birds, as 
well as some 
others (such 
as the bat). 
Others, and 
the majority, 
dwell on the 
surface of the 
earth, mov- 
ing about by 
one means or 
another; for 
on the land 
the habit of 

remaining fixed in one place is not common as it is in the 
sea. An animal that stayed in one place would have little 
chance to obtain its food supply^, for the air is unlike the 
ocean in this respect. A third group of animals occupies 
the ground, either living in it all of the time, or spending 
part of the time on the surface. There are also many of 
the land animals which leave the dry land, either in some 
stage of their development, or from time to time, taking 
to the sea or fresh water for the purpose of obtaining a food 
supply. Among these there are some which spend more 
time in the sea than on the land. 




A view in North Greenland, showing plants and flowers 
(Arctic poppies) peeping above the snow. 



DISTBIBUTION OF ANIMALS AND PLANTS 179 

Among land animals there is much less widespread 
distribution than among the animals of the sea, or the 
plants of land and sea. Some, like birds or insects, move 
so readily that they occupy large areas. For instance, 
ducks and geese \yhich nest in the Arctic, spend the win- 
ter in southern United States and Mexico; but most of 
the land animals move about so slowly, and with such 
difficulty, that they are restricted to relatively limited 
localities. Some species are confined to certain small 
tracts. ^ 

As in the case of plants, so among the animals, there 
is a very great difference between those of the tropics and 
those of the colder zones. The fauna of the humid belt 
of calms, rivals the flora in abundance and variety. The 
tropical forest is alive with creatures of all classes, because 
food is furnished prodigally by the abundant growth of 
vegetation. Where this is more nearly absent, as in des- 
erts, animals become scarce. 

Upon our western plains the limited number of insects and birds, 
contrasts strikingly with the abundance of these creatures in the 
swamps of Arkansas in the same latitude ; and the reptiles and 
higher animals show an even greater diminution. The antelope, the 
prairie dog, a few burrowing animals, one or two species of rabbit, 
the coyote, and a few other species, which constitute the higher ani- 
mal life of the desert, furnish a striking contrast to the scores of 
mammals which exist in more favorable localities. 

In ascending a mountain a similar change is seen, and 
in a journey to the Arctic, one finds the decrease in abun- 

1 For instance, the Australian land, surrounded by water, though not 
far from the large islands to the northward, has animals so different from 
those of other countries, that it may be said to have a fauna of its own. 
Nowhere else in the world are the kangaroo, and the many other strange 
animals of Australia, at present living. 



180 FIRST BOOK OF PHYSICAL GEOGRAPHY 

dance of land animal life parallel to that of the plant life. 
For instance, in central eastern Greenland, with the excep- 
tion of the mosquito, insects are not numerous or abundant. 
Reptiles and burrowing animals are entirely absent, be- 
cause the frost is in the ground throughout the year, and 
in winter the temperature is extremely low. Birds, mainly 
those which obtain food entirely from the sea, are abun- 
dant along the coast in summer, but most of these disap- 
pear with the coming of winter. The chief land birds are 
the snow bunting, ptarmigan, and raven. Reindeer, foxes, 
and Arctic hares are the principal land mammals, and 
these are by no means abundant. 

The polar bear, though coming to the land now and then, lives 
mainly on the floating ice of the sea where he obtains his food. The 
seal and walrus crawl upon the rocks, but only now and then, for they 
spend most of their time in the water or on the sea ice. This list of 
the land animal life of this inhospitable climate furnishes a wonder- 
ful contrast to the thousands of species that inhabit the tropical for- 
ests. Moreover it is noticeable that the higher creatures of the Arctic 
live there only by protecting themselves by a thick covering of fur, 
which keeps out the cold, and that most of them depend not upon the 
land for their food, but upon the sea. In the great ice-covered wastes 
of interior Greenland, there is an entire absence of both animal and 
vegetable life. This is the most absolute desert so far visited by man. 

Distribution of Man. — Once was the time when men 
were distributed in belts, and when races were separated 
and marked by distinct characteristics ; but now, with the 
progress of civilization and the development of means of 
transportation, the barriers have in large measure been 
removed, and before the white race, the others are dis- 
appearing or are being rapidly absorbed. 

There are still savage or uncivilized races which are 
kept within certain bounds by natural barriers, as people 



DISTRIBUTION OF ANIMALS AND PLANTS 181 

were in Europe several centuries ago ; but most races 
have reached the stage of development when natural 
barriers are easily overcome. Rivers no longer present 
serious obstacles to travel, as they did when the bounda- 
ries of some of the European countries were drawn. Seas 
are no longer crossed with difficulty, as was the case when 
England, Scandinavia, and other countries developed in- 
dependently of their neighbors; and mountains are no 
longer such impenetrable barriers as they were in the time 
when it was possible for a tiny state, like Switzerland, to 
exist independently in the midst of greedy neighbors. 

The boundary lines of many countries were drawn in the days 
when even a dense forest was a diflBculty of a serious nature. For a 
long time the Appalachian forests, aided to be sure by the Indian 
occupants, served as a barrier to the progress of the American people, 
and caused a concentration along the Atlantic coast; and it is doubt- 
ful whether the American Revolution would have been successful had 
it been possible for the early settlers to have spread themselves over 
the western territory. 

Without serious study one can hardly realize how closely 
dependent upon geographic conditions has the develop- 
ment of the human race been, although now we have 
nearly risen above this dependence on natural surround- 
ings. In his advance toward a higher civilization, man 
has been subjected to many of the same influences that 
have affected the abundance and variety of plants and 
animals. 

For instance, amid the conditions of the tropics, al- 
though savage, his superior intelligence and skill made 
man the master ; but his mastery cost so little effort, and 
his livelihood was so secure, that he did not advance as 
rapidly as those who were placed amid the greater dif- 



182 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



Acuities of the temperate zone. Here constant effort was 
necessary, so that the intelligence and skill were developed 
which have made the men of temperate latitudes the mas- 
ters of the world, and not merely of other men, but to some 
extent of Nature herself. The people of the Arctic have 

had too severe a struggle, 
and too little opportunity, 
and as a result they have 
developed hardly more than 
the races of the tropics ; yet 
among uncivilized peoples, 
one will probably not find a 
more intelligent race than 
the Esquimaux of the Arctic. 
Modes of Distribution of 
Animals and Plants. — While 
from place to place there are 
many variations in the kind 
of animals and plants, many 
species are widely distrib- 
uted. The same species in 
some cases are found in 
Siberia and British North 
America, or in both eastern and central United States. 
Those animals wliich live in the air or water are most 
easily distributed, being drifted about in these media. 
One may gain an excellent idea of the general subject of 
the modes by which animals and plants are distributed 
over the earth, by comparing the fauna and flora of Ber- 
muda with that of the United States ; for in the resem- 
blances and differences which are found, the chief causes 
for distribution are illustrated. 




Fig. 87. 

A Bermuda road bordered by cedar 
groves. 



DISTBIBUTION OF ANIMALS AND PLANTS 183 

The Bermudas lie about 600 miles from the Carolina coast, which 
is the nearest land. They form a cluster of tiny islands, absolutely 
alone in the sea, and have never been connected with Xorth America ; 
but yet the animals and plants are American in kind. Upon their 
surface we find the cedar (Fig. 87) and other plants from the same 
latitude in Xorth America, the cactus and Spanish dagger, which on 
the mainland exist on the arid plains, the palmetto, and other Bahama 
and Florida plants, the oleander, and scores of other species common 
in southern United States. 

When these islands were first visited, not a single mammal, 
excepting the bat, was found on the island. Insects of the same kind 




Fig. 
A bit of Bermuda landscape. 

as those of the mainland are numerous ; and birds are also there in 
considerable numbers, particularly the ground dove, redbii'd, blue- 
bird, catbird, and a few others. A tiny lizard of the same kind as 
one in the West Indies is also found there. 

How did these come to this remote island, and why are 
there no larger animals ? One of the most striking facts 
concerning the fauna of the Bermudas, is that the animal 
life is chiefly made up of species which can fly ; and every 
year there is proof that it is this fact which accounts for 
their presence. Robins and other birds of passage stop 
upon this land during their annual migrations, and during 
pr after heavy storms, many species of birds are seen which 



184 FIRST BOOK OF PHYSICAL GEOGRAPHY 

do not naturally belong there, and which quickly disap- 
pear. They have been blown out to sea by the wind, and 
the number which have been thus driven away from the 
land, may be shown by the fact, that although the land 
birds native to the island number hardly more than a 
dozen, 185 different species have been found there. Even 
the tiny humming-bird has been seen in the Bermudas.^ 
No doubt for every tiny bird that makes the journey in 
safety, scores perish at sea. Naturally then, the dwellers 
of the air, either by direct flight, or driven by accident, 
may make a journey across the sea, and better yet over 
the land, perhaps starting a colony in some new place not 
before occupied by this particular species. 

Birds, eating fruit upon the mainland, after arriving 
at the island, or in fact after making any journey, may 
drop seeds, which, sprouting, develop into plants that 
start a growth of a new kind in this place. Also the 
wind, carrying lighter seeds, may drive them to far-distant 
lands. The first lizards which came to the Bermudas 
were, no doubt carried there upon bits of floating wood, 
moving in the ocean currents, which have also carried the 
shells now inhabiting the land and the water which sur- 
rounds these islands. None of the larger and higher 
species of animals can take such a journey; for there 
would be nothing to float them, and in any event they 
probably could not survive it. 

1 It may appear strange that so small a bird can make so great a jour- 
ney ; but it must be remembered that there are many logs and bits of wood 
floating in the sea, and that these will serve as a resting-place for the 
birds that are forced to make this flight against their will. It is by no 
means uncommon, when sailing far out of sight of land, to see some small 
land bird wearily flying toward the ship, where he rests for awhile in 
the rigging, before taking up his flight in the effort to again reach the lan(J 
from which he has been driven. 



DISTRIBUTION OF ANIMALS AND PLANTS 185 

These are the means by which animals and plants are 
distributed: some make direct journeys, some come by 
chance, and some are carried by accident. On the land 
they move slowly along, spreading out into whatever new 
territory they may be able to occupy. 

This process of extension of species into areas previously unoccupied 
by them, is well illustrated in the case of many of the weeds, such as 
the field daisy, introduced by chance into this country during the 
Revolution, and now one of the commonest of plants; or of the 
Canadian thistle, which has extended its range so as to become a pest 
in farming districts. Many insects, like the Colorado beetle or potato 
bug, and other animals, like the English sparrow, the latter a European 
species, finding themselves in a new region favorable to their develop- 
ment, have multiplied and spread in a wonderful manner. In Australia 
the rabbit, introduced from Europe not many years ago, has become 
a national pest. 

Man has now come upon the scene, and has become the 
most potent of all agents in the distribution of animals 
and plants. Formerly, by their own or by accidental 
movements, organisms had spread about over the land, so 
that the world was divided into fairly definite zones, each 
species having found a place for itself and occupying a 
restricted area; but man is interrupting all this, killing 
off species here, and introducing them there, so that very 
often it is difficult to say which species are native and 
which introduced. For instance, in Bermuda, many 
plants carried there by man have taken such a footing on 
the islands that in some cases it is almost impossible 
to say that they were not there before man came. 

Barriers to the Spread of Life. — The most effective bar- 
rier to the spread of life is temperature. No matter by 
what means the cocoanut or the coffee berry were carried 
to northern United States, they could not grow ; nor would 



186 FIRST BOOK OF PHYSICAL GEOGBAPHT 

one of our pines or spruces find the tropical belt a con- 
genial home. There is therefore, on land as well as sea, 
a limit to the distribution both of animals and plants, 
dependent largely upon temperature. Therefore the spe- 
cies of the north temperate and Arctic belts are not at 
all the same as those of the southern hemisphere, even 
where the climatic conditions are the same, for they cannot 
pass the great tropical harrier. 

For the same reason mountain ranges serve as a partial 
barrier; for because of the low temperatures, many species 
cannot cross them, though some of the more hardy species 
are able to make the journey, and others pass around the 
ends, so that these elevations are not complete barriers. 
Organisms accustomed to life in a humid climate cannot 
survive on a desert^ and therefore this also serves as a 
partial barrier. Even a river^ or a chain of lakes, may 
mark the limit of spread of some species ; and sometimes 
a forest, or an open country/, may serve as a barrier; for 
the forest-dweller may not be able to endure a journey 
across the open prairie, or the inhabitant of an open plain 
may find the passage through a forest impossible. 

However, the great barrier is the sea, and this has been 
well illustrated in the case of Bermuda. Only certain 
forms can make the passage of this barrier, and therefore 
the land fauna and flora of oceanic islands may differ from 
those of the nearest mainland by the absence of many 
species, especially of the higher animals, and oftentimes 
by the presence of new forms, which have been developed 
there, and have not yet spread to the mainland. In the 
far-away islands of the mid-ocean these peculiarities are 
very marked. 



PART III.— THE OCEAN 

CHAPTER XII 

GENERAL DESCRIPTION OF THE OCEAN 

Area of the Ocean. — Upon the earth there are about 
145,000,000 square miles of ocean water, covering and 
hiding from view about three-quarters of its surface. It 
is not a uniform sheet, but is irregularly distributed 
in oceans, and between these there are continents, while 
above its surface there rise numerous islands. Its waters 
bathe the shores of these lands, and the contact between 
land and water is very irregular, especially in the northern 
hemisphere, where the land is indented by many bays and 
harbors, and even by great enclosed seas. This line of 
contact is the scene of many changes (Chap. XVIII), 
for the waves of the sea are incessantly at work in an 
attack upon the land. 

Importance of the Ocean. — The ocean surface is now 
traversed by ships, carrying the products of one zone to 
the people of another; but at one time, before our means 
of navigation had reached such perfection, the sea surface 
was a barrier to the spread of man, almost as effectual as it 
now is to animals. Then distant lands were unknown, 
and even short journeys by sea were hazardous. 

The water of the ocean moderates the climate of the 
globe, and especially influences the land near by ; it f ur- 

187 



188 FIRST BOOK OF PHYSICAL GEOGRAPHY 

nishes the air with the vapor which falls as rain, and 
therefore supplies our streams and lakes with water, which 
after a passage over the land, may return to the parent 
sea which gave it birth. The rivers not only take water 
to the ocean, but also carry sediment from the land, and 
so the sea becomes the dumping ground for the waste of 
the land. This, added to that which is wrested from the 
coast by the waves, is strewn over the bed of the sea near 
the land. Last, and by no means least, the ocean is the 
home of myriads of animals, many of which serve man 
as an important source of food. 

The Ocean Water is Salt. — Unlike most of the water of 
the land, the ocean water is distinctly salt, and there is a 
great difference in the percentage of this substance pres- 
ent in it. On an average the amount of salt in the sea 
varies from 3.3% to 3.7% of the whole, so that more than 
96% of the ocean is pure fresh water. Yet so great is the 
bulk of sea water on the earth, that the total amount of 
salt dissolved in it, if deposited in a layer over the sur- 
face of the land, would make a bed over 400 feet thick. 

In rainy belts, such as the doldrumsj the constant supply of rain 
freshens the sea water; and so also, the ocean is less salty at the 
mouths of large rivers, and near such great glaciers as those of Green- 
land, which enter the sea and furnish fresh water when the ice melts. 
On the other hand, where evaporation is rapid, as in the trade-wind 
belt, the sea water becomes salter than elsewhere ; for evaporated 
water is fresh, and when it leaves the ocean, the salt remains behind, 
thus making the surface water that remains still salter. Salt water 
is heavier than fresh, and we say that it is more dense. Calling the 
density of fresh water 1, sea water has an average density of 1.02; 
but as its saltness varies, the density likewise changes. 

Besides the common salt which gives the ocean water 
its taste, there are minute percentages of other solids in 



GENERAL DESCRIPTION OF THE OCEAN 189 

solution. The most important of these is carbonate of 
lime, the material out of which corals build their skeletons, 
and shellfish their shells. They take it with their food 
and transform it to the solid forms which we know so 
well, just as the land animals take with their food mate- 
rials which they build into bone, teeth, etc. 

In addition to these solid substances, the sea water car- 
ries quantities of oxygen and other gases of the atmosphere, 
and the fishes take this from the water by means of their 
gills, very much as we take oxygen from the air by means 
of our lungs. Without it they would die. Some of the 
oxygen present in the surface water is furnished by plants, 
but much is absorbed from the atmosphere, or carried into 
the sea by rain and river water. 

No one is able to say just what is the source of the salt in the ocean. 
Probably the sea has always been salt, having become so when first the 
waters gathered on the surface of the globe. If the explanation of the 
origin of the earth which the Nebular Hypothesis furnishes is cor- 
rect, the sea in the early history of the globe must have been impure 
with many substances, for then the earth was hot. As the oceans de- 
scended from the atmosphere, in the condition of heated rain, to form 
the great bodies of water of the globe, they must have contained salt 
and other substances in solution. Upon this explanation much of the 
salt now contained in the ocean was then furnished it ; but all rivers 
that flow over the land carry salt, which they have obtained from the 
rocks and soils, and so the sea is probably becoming salter all the 
time, just as some lakes without outlet are even now being trans- 
formed to salt seas. 

Temperature of the Ocean Surface. — Naturally the tem- 
perature of the surface of the ocean varies from place to 
place, for it is warmed by the sun in a manner similar to 
the warming of the land. Near the Equator the constant 
warmth produces warm ocean water, and in the Arctic 



190 FIBST BOOK OF PHYSICAL GEOGBAPBT 

and Antarctic seas, on the other hand, the water is cooled 
by the constant coolness of the climate. However, the 
temperature of the sea never descends below the freezing- 
point of salt water, 1 but when this is reached the water 
congeals and sea ice is formed. Up to this point the 
water sinks as it cools, for as in the case of air, cooling 
makes it more dense, and therefore causes it to settle. 

The sea-made ice of the Arctic, or the jloe ice, as it is called when 
drifting about, forms over the greater portion of the water which sur- 
rounds the poles. Far to the north, the surface of the ocean in winter 
is transformed to solid ice, very much as ponds are in the winter; 




Fig. 89. 
Arctic sea ice, northern end of Labrador. 

but the surface of this is very much rougher, for the winds, breaking 
the ice, pile the blocks one upon another, and the tides and currents 
moving it hither and thither, crack and break it into blocks, which 
are sometimes flat, but commonly raised one upon another, so that 
in many places the Arctic ice is so rough that travel over its surface 
is almost impossible. The depth of this sea ice is generally not over 
10 or 20 feet, though sometimes greater than this. 

Since the ocean waters are in movement at the surface, this floe 

1 This varies with the density of the sea water, but is generally between 
28° and 29°. 



GENERAL DESCRIPTION OF THE OCEAN 191 

ice is carried about; and as the movement of the water is mainly 
toward the south, the accumulation of the winter is in part removed 
to warmer latitudes during the summer. Both during winter and 
summer, there is a movement of the sea ice in this direction. Hence 
it does not increase in thickriess as winter succeeds winter, but some 
goes off to other regions, where, owing to the warmer climate, it melts 
and disappears. There is a constant procession of this ice past the 
shores of Baffin Land (Figs. 72 and 89), and in spring and early 
summer it extends along the entire coast of Labrador, and even as 
far as Newfoundland. 

Tlie difference in the temperature of the sea causes 
movement to start, very much as the air moves by convec- 
tion. The warmer water of the tro]3ics is made light 
by the higher temperature, and hence it floats, while 
that of the colder regions, being denser, settles, and 
warmer water takes its place. This is one of the rea- 
sons why the ocean waters are in movement in the form 
of ocean currents, and it is one of the reasons why the 
surface water changes temperature so slightly ; for with 
a difference in temperature, and hence in density, the 
water moves about, endeavoring to equalize the differ- 
ences. 

Life on the Bottom (Chapter XI). — For a long time 
almost nothing was known about the bed of the sea, an 
area equal to about three-fourths of the earth's surface. 
It was supposed to be a great barren zone uninhabited by 
life. People knew that the pressure on the bottom must 
be tremendous, and that probably no sunlight passed 
through the great depth of water, and these peculiarities 
were thought to be sufficient reason for preventing the 
existence of animals in the depths of the sea. 

When it was found to be possible to lay cables across 
the ocean, it was discovered that animals did live in this 



192 



FIRST BOOK OF PHYSICAL GEOGBAPHT 



great zone, and an interest was aroused among scientific 
men, as a result of which explorations of the sea bottom 
were undertaken, partl}^ to study animal life, and partly 
to determine the outline of the ocean bottom, the latter 
point being necessary in some cases in order to find if it 
would be possible to lay cables upon it, and in order to 

select the best lines. It is 
now known that animals 
live there notwithstanding 
the coldness of water, the 
darkness, and the great pres- 
sure; and it is also known 
that they obtain their food 
mainly from the death of 
the creatures with which the 
surface waters teem. 




)CO0\ 



73" 



That there is great pressure on 
the bottom of the ocean is proved 
by the fact that fishes brought to 
the surface often have their skin 
cracked, their eyes protruding 
from their heads, and their air- 
bladders from their mouths. 
While they are on the bottom 
there is a tremendous pressure ; 
Fig. 90. but it is exerted on every part of 

Deep-sea sounding machine, with and their bod}^, just as an air pressure 
without the sinker ^f ^i^^^^^ 15 pounds to the square 

inch is exerted on every part of 
our own bodies. But when these creatures are raised to the surface, 
the pressure is removed from the outside, and that from within, 
pressing outward, gives the results mentioned. No doubt the animals 
move about in the waters of the deep sea with the same ease that 
their fellow-creatures do in the waters at the surface. 



GEN HEAL DESCRIPTION OF THE OCEAN 193 

Methods used in Studying the Ocean Bed. — In the study of the ocean 
bottom there are several sets of facts which are chiefly sought. It is 
desired to know what animals inhabit these deep recesses, what the 
temperature is, how deep the water is, and hence something about 
the outline of the ocean bed. In carrying on the study, the first thing 
to be learned is the depth. 

The sounding is made by means of a small steel wire which is 
reeled off from a sounding machine (Fig. 90). It is necessary that 
this shall be small, for the weight of a coil of heavy wire, and its 
friction in the water, would make it difficult to draw back to the 
surface what had been lowered. On the end of the sounding wire 
there is a heavy iron ball which sinks to the bottom, and when it 
strikes sends a shock through the wire, which causes a spring attached 
to the machine to jump, and then the reeling of the wire from the 
wheel is stopped. So delicately made is this machine, that the depth 
is measured with an error of only a few feet, and we now have many 
thousand such soundings in different parts of the ocean. 

The sounding wire is so frail that it would be impossible to draw 
the weight back to the surface from the great depths, and hence it is 
left on the bottom. By doing this it is made certain that the ocean 
floor has been reached. The iron weight, which is a cannon ball 
pierced by a hole, surrounds a cylinder,^ at the top of which is a joint, 
which when the ball touches bottom, bends and releases a small hook 
upon which the cannon ball has been suspended by a wire. When 
this hook drops down, — and it cannot do so until the bottom is 
reached, — the ball is released, and hence remains on the sea bed, 
while the wdre and water bottle are drawn to the surface. 

At the same time the temperature of the water is obtained. A 
thermometer is attached to the sounding wire near the water bottle, 
and others at different points between the surface and the bottom. 
These are so constructed that when the sounding wire is drawn in, 

1 This is the xoater bottle^ which by automatic contrivances remains 
open on the way down, and becomes hermetically sealed by means of a 
tiny screw which revolves when the wire is being drawn up. Therefore 
the water bottle brings to the surface a sample of the water from the 
ocean bottom. It is worthy of mention that this water is charged with 
gases under great pressure, so that when the bottle is opened at the sur- 
face, the water escapes as soda water does from a fountain. 
o 



194 



FIBST BOOK OF PHYSICAL GEOGRAPHY 



they turn upside down, being allowed to do so by a little screw wheel 
which is unscrewed by the upward movement through the water. 
When the thermometer overturns, the temperature at that particular 
time is recorded by the height of the mercury column, and this is not 
afterwards disturbed unless the instrument is turned right side up 
again. Therefore the temperature for any depth may be obtained.^ 
After a sounding has been made, a dredging is often undertaken 
for the purpose of obtaining some of the deep-sea animals. The 




Fig. 91. 
Lowering a deep-sea dredge to the ocean bottom. 



dredge or deep-sea trawl is an iron frame with a long bag net attached. 
This is lowered by means of a strong wire rope, and then dragged 
over the ocean bottom, taking whatever chances to come in its wa;^, 
and a dredge rarely comes from the bottom without containing some 
of the deep-sea creatures. It is necessary that the frame shall be 
dragged over the bottom, and to do this, much more rope is needed 

1 Other facts are also obtained, one of importance being the determina- 
tion of the nature of the materials covering the bottom. This is done by 
placing some soft substance, like soap, on the bottom of the water bottle, 
and to this the mud or sand of the sea floor will cling and be brought up 
to the surface. 



GENERAL DESCRIPTION OF THE OCEAN 



195 



than in sounding, for if an extra amount were not allowed, when the 
steamer began to drag the dredge it would simply be towed through 
the water. Sometimes a weight is attached to the rope in order to 
cause it to sag (Fig. 91) ; but in real deep water the great amount of 
heavy wire rope used is sufficient for this purpose. 




35-36 



Ex planat ion. 

36-37 ■ 37-3810 38-39V,H +39V.n 



Fig. 92. 
Map showing temperature of the bottom of the north Atlantic. 



Ocean Bottom Temperatures. — As might be expected, 
tlie water of the bottom of the sea is cold. Excepting pos- 
sibly in the Arctic seas, the temperature of the water de- 
creases as the depth becomes greater. This is because the 



196 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



sun warms only the surface layers; and whenever, either 
during the winter or the night, the water at the surface is 
cooled, it sinks, because it then becomes heavier. So there 
is a settling of the cold water, and therefore the tempera- 
ture at the bottom of the frigid seas is about at the freezing 
point of salt water. Between Iceland and Norway the 

ocean bottom 
temperature is 
below 30°; but 
similar cold wa- 
ter is also found 
at the bottom 
near the Equa- 
tor; and over a 
great part of the 
sea floor, in its 
deepest portions, 
the temperature 
is between 32° 
and 35°, even 
within the trop- 
ics. Manifestly 
this cannot be 
due to the sink- 
ing of water in the warmer zones, for the temperature 
of the sea surface within the tropics is rarely below 70° 
(Plate 14). There are various reasons for believing that 
these cold waters have come to their place as a result of the 
slow movements of ocean currents along the ocean bottom, 
from the frigid zones toward the Equator (Chap. XIII). 

Generally the temperature of the ocean descends rather 
rapidly just below the surface (Fig. 93), particularly in the 




Fig. 93. 

A section of the ocean from New York to Ber- 
muda, showing the temperature at various depths 
(fathom = G feet) . 



GENEBAL DESCRIPTION OF TEE OCEAN 



197 



tropical zone, where the upper layers of water are warm ; 
but after passing from this upper zone, the temperature 
descends more slowly. At first, a depth of a few hundred 
feet makes a difference of several degrees, and then this 
rate becomes less, until near the bottom there may be a 
change of not more than 1° in a thousand feet. In other 
words, the ocean water is stratified into layers having 
different temperatures, the highest at the top and the 
lowest at the bottom. Therefore, as a general rule, the 
greater the depth, the lower the temperature. 











OCEAN SURFACE 






u. 

O Z 

1^ 




1000 FATHOMS 


GULF OF MEXICO 
39.5° _^^^ 


39.5^,,„-^^^ 


^^ 


^5" 


ATLANTIC OCEAN 
^\^ 2000 FATHOMS 


• -^ \35° 1 



Fig. 94. 

Section of part of Gulf of Mexico and Atlantic Ocean, showing depth and 

temperature. 

There are several exceptions to this, most of which are 
found in such partly enclosed seas as the Mediterranean 
and Gulf of Mexico (Fig. 94). In the latter, for instance, 
the temperature decreases normally until it reaches about 
39.5° ; and then there is no further decrease, while outside, 
in the open Atlantic, where the ocean depth is no greater, 
the temperature decreases to 35°. Such a difference must 
have a special explanation; for if there were chances for 
free circulation, the cold water of the deeper Atlantic 
would flow in and displace the warmer water that over- 
spreads the deeper parts of the Gulf of Mexico. The ex- 



198 FIBST BOOK OF PHYSICAL GEOGRAPHY 

planation for this peculiarity is found in the fact that there 
is a rise in the sea bottom which makes the bed of the Gulf 
a basin with the rim higher than the centre. The coldest 
water that can enter is that at the level of the top of the 
rim, which in the open Atlantic is about 39.5°. 

The same condition exists in the Mediterranean, where 
the temperature of the bottom at a depth of 12,000 feet is 
only 55°, while in the open Atlantic at the same depth 
it is 20° lower. The temperature at the bottom of the 
Mediterranean is the same as that in the Atlantic at the 
same level as the bed of the Strait of Gibraltar. 

The Depth of the Sea. — The deepest part of the sea so 
far discovered lies to the south of the Friendly Islands, 
which are in the south Pacific, east of Australia. There 
the depth is over 5000 fathoms, or 30,000 feet, more than 
5J miles, being greater than the elevation of the highest 
land above the sea level, which is about 29,000 feet. The 
deepest known point of the Atlantic is 4561 fathoms, within 
70 miles of the island of Porto Rico. Not only are parts of 
the ocean deeper below the sea level than the highest land 
rises above it, but its average depth is very much greater. 

Near the continents, and in some of the partly enclosed 
seas, the ocean is not very deep; but over nearly its entire 
area, beyond a score or two of miles from the land, and 
frequently much nearer, the depth is a mile or two, and 
very often more. Surrounding most of the continents 
there is a shelf of varying width, from a few miles to over 
100 miles, over which the sea is shallow; but beyond this 
the depth rapidly increases, until the great ocean abysses 
are reached. The best way to gain an idea of this differ- 
ence is to make two sections, one from New York to 
Bermuda, the other from New York to Great Britain. 



GREV DEEP /!;i 



FATHOMS 
0-100 



100-500. _. 
500-1000, 



Facing page 19 




Facing page 108. 



Plate 15. 
Map showing depth of the sea in the Atlantic Ocean. 



GENEBAL BESCBIPTION OF THE OCEAN . 199 

Starting from western New York at an elevation of 1000 or 2000 
feet, and passing over undulating ground, we come to the seashore, 
where by a further moderate descent, the land passes beneath the sea 
and the depth of the water gradually increases (Fig. 95). Twenty- 
five miles away the depth is perhaps 200 or 300 feet, and the water 
continues to deepen very gradually, until at a distance of about 75 
miles from ISTew York the depth is 100 fathoms, or 600 feet. Between 
jSTew York and this point, the temperature of the ocean-bottom water 
varies with the season, being warm in summer and cold in winter ; 
but at the outer limit the temperature is always somewhat high, being- 
kept so by the influence of the water of the Gulf Stream. 

This zone of shallow water rests upon the continental shelf; and 
going further we find the depth of the water rapidly increasing, so that 




Fig. 95. 
Section of ocean from New York to Bermuda, showing depth and temperature. 

within a few miles from the edge of the shelf, the depth has increased 
from 100 fathoms to 1000 fathoms, or more than a mile. This region 
of steep slope, which borders the entire continent at varying distances 
from the coast, is called the continental slope, and the descent upon its 
face is about as rapid as that of a moderate mountain slope. Along 
this the temperature rapidly decreases, until at the depth of 1000 
fathoms it is 38°. Then the depth of the sea becomes gradually 
greater, until the deepest point of over 3000 fathoms is reached, well 
toward the Bermudas. Here the temperature of the sea is 34°. 

Within sight of the Bermudas, not more than 30 miles away, the 
bed of the sea begins to rapidly ascend, and in this short distance 
rises more than two miles, so that if one could stand on the sea floor 
30 miles from the Bermudas, he would see a lofty conical mountain, 
quite like a volcano in form. As the depth becomes less on the sides 
of this cone, the temperature increases, at first slowly, then rapidly, 
until the temperature of the surface, about 70°, is reached. On the 



200 FIBST BOOK OF PHYSICAL GEOGBAPHY 

opposite side the descent is similar to this, and soon the cold, dark 
depths of the sea are again found. 

Passing from New York toward England, the first part of the 
journey would be similar to that just described. The great ocean 
depths that are reached beyond the edge of the continental shelf are 
nowhere interrupted by shallows, but continue until near the mid- 
Atlantic,- where the bottom rises in a great, broad swell, forming a 
ridge which divides the Atlantic in a somewhat disconnected way 
from the northern part to the south Atlantic. Generally the depth 
of this mid- Atlantic ridge is 1000 or 2000 fathoms, but the water is 
shallower here than on either side, and upon its crest the temperature 
is higher than in the greater depths to the east or the west (Plate 15). 

Beyond this, toward Em^ope, the bottom again descends, reaching 
a depth of nearly 3000 fathoms, and then, at a distance of 50 miles 




Section to show, in diagram, the conditions of temperature and deptli in the 
Atlantic. Depth and width of continental shelf greatly exaggerated. 

from the British coast, the continental slope rises steeply, as on the 
American side ; and with this rise the temperature increases. Then, 
from the crest of this to the shores of England, the water, at first 
about 100 fathoms deep, becomes gradually shallower ; and here, as 
on the American side, the temperature changes with the seasons. 

Topography of the Ocean Bottom. — These two sections 
are characteristic of the ocean floor in various parts of the 
world. Bordering the continents there are plains covered 
with very little water, and varying in width, terminating 
in steep slopes facing toward the sea, beyond which are 
extensive plains covered by deep water and extending over 
nearly three-fourths of the earth's surface. These plains 
of the ocean bottom are the most extensive in the world, 



GENEBAL DESCRIPTION OF THE OCEAN 



201 



forming great monotonous expanses of ocean-bottom clay, 
the surface of which no doubt rises and falls in gentle 
swells and is here and there relieved by single peaks, like 
the Bermudas, or groups of peaks, like the Hawaiian 




Fig. 97. 

A part of the Jones Model of the earth, showing a part of the ocean bed and 
the continents and islands. Copyright, 1894, by Thomas Jones, Chicago, 111. 

Islands, some rising to the surface, and some not reaching 
so far. Here and there also there are mountain ranges, 
like the chains of islands forming the East and West 
Indies; and in some parts of the sea, as in the south 



202 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Pacific, numbers of disconnected peaks rise from. the great 
ocean-bottom plain (Fig. 97). 

There is a distinct difference between the outline of the 




Fig. 98. 
North and South America in relief with neighboring ocean beds — showing 
continent elevations, mountain ranges, and ocean basins. Copyright, 1894, i 
by Thomas Jones, Chicago, 111. I 

sea floor and that of the land. In both there are volcanoes | 

and mountains, and in both there are plains and plateaus ; ' 
but on the land the action of the weather and the running 

water have gullied and carved the surface into an exceed- I 



GENERAL DESCRIPTION OF THE OCEAN 203 

ingly irregular outline, not only with great elevations and 
depressions, but also with minute hills and valleys. 

On the sea bottom however, these agents of land ero- 
sion are absent, and the plains and plateaus are level, 
while the mountains and volcanoes rise steeply, with 
smooth, uncut sides. Not only are the causes for the land 
irregularities absent, but there is also a constant rain of 
sediment, some brought to the sea by rivers, some wrested 
from the shore by waves, and much formed by the death 
of animals which have taken carbonate of lime from the 
water and built it into shells and skeletons, which when 
they die fall to the sea bottom and accumulate. This 
steady supply of materials, distributed over the sea floor, 
tends to smooth out all the smaller irregularities which 
naturally exist. For these reasons the main feature of 
the sea bottom is levelness, broken here and there by 
steeply ascending and smooth-sided peaks and mountain 
chains (Plate 15 and Figs. 97 and 98). 

The Ocean Bed. — The land surface is either bare rock 
or soil. The sea bed is nearly everywhere soft clay or 
muddy ooze. Near the land, gravel and sand are dis- 
tributed over the bottom, being furnished by rivers and 
waves ; but these fragments are too heavy to be carried far 
from the coast, and they therefore settle near the shore, 
so that the further we go from it, the finer are the mate- 
rials covering the sea bed. Even 100 miles from the coast 
there are some minute bits of clay floating in the surface 
waters. 

Globigerina Ooze. — But in the open sea these materials 
settle to the bottom in very much smaller quantity than 
the shells of the minute and almost microscopic animals 
which live in such abundance in the surface waters, and 



204 FIRST BOOK OF PHYSICAL GEOGRAPHY 

which upon their death sink to the bottom. Therefore 
over most of the sea floor the bed is made of an ooze, 
chiefly composed of remnants of these shells. 

Among the shell-bearing pelagic animals some of the most com- 
mon are species belonging to the genus Globigerina ; and hence over 
great areas this makes an accumulation of what is called Globigerina 
ooze, which is somewhat like the chalk of England and France. Since 
they have been falling to the sea bottom in the open ocean for ages 
past, they must have formed a great thickness of this ooze, though to 
make a layer a foot in depth must require many centuries, since each 
grain represents the life and death of an animal whose size is less than 
that of a pin-head. In parts of the ocean other minute species, such 
as Diatoms and Infusoria, are more common than the Globigerina ; 
and on the bottom of these seas the ooze is called diatomaceous or infu- 
sorial, according to w^hich form predominates. 

Red Clay. — Covering even a larger area than this (about 51,000,000 
square miles) , is a still more remarkable deposit called red clay. This 
is a red mud occurring in the deeper parts of the ocean, below the 
depth of 2000 or 2500 fathoms. In it are found bits of meteoric iron, 
representing the partly burned-up meteors, fragments of pumice, and 
the more indestructible parts of sea animals, such as the teeth, which 
are less easily dissolved than the carbonate of lime which forms the 
shells. Because of the presence of gases (especially carbonic acid 
gas) held under the great pressure which exists there, the shells 
have been dissolved in the sea water. 

Every shell contains something else than carbonate of lime, such 
as minute quantities of iron and silica, which are not so easily dis- 
solved as the carbonate of lime. Hence, while the latter is taken into 
solution, these remain behind ; and it is of this insoluble residue that 
the red clay is formed, its color being due to the presence of the iron. 
Therefore, while the Globigerina ooze is very slowly formed by the 
accumulation of many minute shells, the red clay is gathering with 
infinitely greater slowness as a result of the accumulation of minute 
remnants of these tiny shells. 



CHAPTER XIII 

THE MOVEMENTS OF THE OCEAN 

Wind Waves. — A slight wind disturbs the surface of 
the sea, causing ripples to start on the water. Thus the 
surface rises and falls as one ripple succeeds another, pass- 
ing in the direction toward which the wind is blowing; 
but an object floating in the water does not move along as 
fast as the tiny wave does. From this it is seen that the 
wave is not a hodily forward movement of the water, but 
a disturbance of its surface, just as a series of ring waves 
move outward in all directions from the centre, when a 
stone is thrown into the water. The wave form consists 
of two parts, an elevation and depression, the top or ele- 
vated part being called the crest of the wave, while the 
depression between two crests is called the trough. 

The cause of the wave is the friction of the wind on the 
water, just as we may raise a tiny wind wave in a basin 
of water by blowing over its surface. This friction not 
only causes the water to rise and fall, but it really does 
drive a very small amount forward, so that a floating body 
not merely rises and falls as the wave passes under it, but 
actually floats slowly forward. This surface movement of 
the water constitutes a gentle current, a wind drifts at the 
very surface. Therefore everywhere that wind is blowing 
over water, there is a gentle current moving slowly along 
before the wind. 

205 



206 FIRST BOOK OF PHYSICAL GEOGRAPHY 

If the wind continues, and especially if it freshens, the 
waves become higher, for the cause is increased because 
then there is more friction. ^ In addition to this, the 
height of the wave is increased as one wave catches up 
with another, so that two combine to form one higher than 
either of the others. This increase may continue until 
waves reach "mountainous height," which is an exagger- 
ation due to the great apparent height of the wave seen 
from a small ship.^ 

When a wind wave attains these dimensions, its effect is felt to a 
depth of 200 or 300 feet, and it then becomes such a powerful move- 
ment of the water that it may last for a long time after its cause has 
disappeared. If one throws a stone into the water, the ripples which 
it starts extend perhaps for scores of feet, and gradually die out; and 
so it is with the great ocean waves. jS'ot uncommonlj^, on a perfectly 
calm day, when the water surface is smooth and glassy, it heaves with 
great sicells or rollers, which have originated in some distant part of 
the sea, and have passed far beyond the place in which they were 
formed. 

In the open ocean the waves are usually doing little 
work excepting to cause the surface to rise and fall. 
Vessels pass over them, being lifted and lowered as the 
waves pass by, but not being injured, excepting rarely, 
when in violent gales the surface of the sea is lashed 
into a broken mass of whitened wave crests between deep, 

1 This may be illustrated by blowing gently on the surface of the water 
in a basin, and then blowing with much greater force. 

2 Large ocean waves rarely rise more than 20 or 30 feet from trough to 
crest, but some have been measured which rose 46 feet, and it is believed 
that some reach the height of 60 feet from the lowest point of the trough 
to the highest part of the crest. Those having a height of 40 or 50 feet 
travel as much as 45 miles per hour, and the distance from the crest of 
one wave to that of another may be over 700 feet ; but one may travel on 
the sea for a long distance without finding such huge disturbances. 



THE MOVEMENTS OF THE OCEAN 207 

narrow troughs. Then small vessels, tossed violently 
about, are sometimes foundered, and larger ones at times 
seriously damaged. When they come to the coast, the 
waves change their habit, and dash upon the exposed 
shores with resistless fury. 

On the coast the wave form is destroyed, for as the 
water becomes shallower, the great waves have their move- 
ment interfered with, and the motion of the bottom part 



_A» DIRECTION OF WAVE MOVEMENT foru- 




Fig. 99. 
Diagram to show approach of a wave upon a beach. 

is partially checked as it comes in contact with the sea 
floor. The upper portion of the wave, being less checked 
in its movement, progresses, so that as a result of this the 
crest gradually changes, first becoming steeper on the land 
side, and then falling forward in the form of a breaker^ 
which rushes violently upon the coast, no longer as a mere 
wave movement, but as an onward flood of water, furi- 
ously hurled against the coast. Upon the beach the surf 
rushes far above the average water level; and against the 
cliffs the waves strike a blow which causes the rocks to 
tremble, and sends a roar through the air, while the spray 
dashes high upon the coast. 

During these times of violent waves there is great work 
of destruction being done. The sand of the beach is 
washed backward and forward, or the pebbles are rolled 
about, causing a deafening roar as they are ground together. 
This constant grinding wears them slowly away, rounding 



208 FIRST BOOK OF PHYSICAL GEOGBAPHY 

them and finally reducing them to bits of clay or sand.^ 
The beaches are the mills in which the rock fragments, 
driven ashore by the waves, or wrested by them from the 
rocky headlands, are ground to form the sediment which 
is being strewn over the ocean bed. The violent waves 
not only work here, but also against the cliffs of hard 
or soft rock, which they are also wearing slowly away. 
Sometimes the power of the waves becomes so great, that 
blocks tons in weight are wrested from their beds and 
moved along the coast. 

But if the waves merely destroyed the coast, they would 
soon lose their power to cut into the land, for as the cliffs 
crumbled, and the debris accumulated at their base, they 
would be protected from further attack, and the waves 
would expend their energy on the fragments which they 
had wrested from the land. Some of these must be 
removed^ so as to leave the cliffs open to the further 
attack of the waves. As the boulders and pebbles are 
ground to pieces by the waves, particles are worn off which 
are so fine that they can float away in the moving water. 
The tidal currents help in this removal, and so also do 
the wind-formed currents of surface water which move 
before the winds. 

Then also, when the waves come upon the coast diago- 
nally, as they often do, the surf, instead of running di- 
rectly along the coast, passes not only toward the land, 
but for some distance along its margin^ so that as wave 
follows wave, the sand and pebbles are washed along the 
shores in the surf. Fragments are thus carried over con- 

1 So rapid is this work of destruction that bricks which have been 
washed ashore upon the beacli, are ground to tiny pebbles in the course of 
a few years. 



THE MOVEMENTS OF THE OCEAN 209 

siderable distances in the direction toward which the 
waves move. Also when the waves run upon the beach 
in the form of surf, the water must return to the sea. 
This return movement, which begins when the wave has 
worn itself out against the shore, and is interrupted when 
the next wave comes, continues below the immediate sur- 
face in the form of a current along the bottom. This, 
which is known as the undertow, is an outward-moving 
current, flowing with such force that bathers are some- 
times caught in it and drawn down and held near the 
bottom until life is extinct.^ 

By these several means the particles which the waves 
take from the coast are slowly ground up and carried 
away, some out to sea, where they settle in the more quiet 
water, and some along shore, until an indentation is 
reached, where, being driven into the more quiet water of 
the protected bay, it settles to the bottom. While it is 
necessary for much of the material to be carried away, in 
order that the waves may continue their attack upon the 
land, it is also important that some fragments should also 
remain in their grasp ; for the work of the waves is made 
effective, not so much by the direct action of the water, as 
by the battering of the pebbles and sand which they hurl 
against the shore. These are the tools with which the 
wave does its work. 

This attack of the waves produces different results 
according to their violence, the exposure of the coast, its 
form, or its rock structure ; and it is performing a great 
work of change, as a result of which the coasts of the 

1 When swimming in the ocean surf it is necessary to keep near the 
surface, and if the feet are allowed to sink toward the bottom the entire 
body may be drawn under. 



210 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



world are not only varying in outline, but are being con- 
structed into certain definite forms. The study of these 
changes may be deferred until we have gained a little 
more knowledge of the land (see Chap. XVIII). 

The Tides : ^ Nature of the Tides. — In most parts of the 
ocean the water surface rises twice each day. The water 
slowly advances, and a person standing upon the coast is 
driven from his position as it rises. Then for a little 

more than 6 hours, it retires as 
slowly as it came, when it again 
begins to rise; and this is re- 
peated again and again, so that 
every 12 hours and 25 minutes 
there is a high tide, with low 
tide between. The rising tide 
is called the flow; the falling, 
the ehh. If one watches the 
tide with care, he sees that it 
does not rise to the same level 
day after day, but that there is 
considerable difference in its 
height. 

Again, from one place to an- 
other there is a variation in the 
height to which the tide rises. 
At Key West, and on oceanic islands, the rise is only 2 
or 3 feet; in Hudson Straits, north of Labrador, it reaches 
an elevation of 30 feet; in parts of the Bay of Fundy the 
high tide is often 30 or 40 feet above the low, and in 




Fig. lOd. 

Diagram to illustrate the dis- 
tortion of the ocean by the 
attraction of the moon, the 
distortion being of course 
greatly exaggerated. 



1 From necessity this subject is treated briefly here. The teacher who i 
wishes to expand the subject will find some material for this in my Ele- ' 
mentary Physical Geography, as well as in other books. i , 



THE MOVEMENTS OF THE OCEAN 



211 



some places is said to reach 50 or 60 feet. In Ungava 
Bay, in the northern part of the Labrador peninsula, the 
tide rises about as high as it does in the Bay of Fundy. 

Causes of Tides. — ■ From an examination of the tides 
it is evident that they are waves of rising and falling 
water, and we know 
that all ocean shores 
are disturbed by them. 
The tides reach greater 
height in V-shaped 
bays than on exposed 
oceanic islands, and 
this is evidently due 
to the influence of the 
land. When a wind 
wave approaches the 
beach, it is caused to 
change its habit and 
to reach higher than 
the natural level of the 
sea. It is piled up; 
and so is the tide wave 
as it enters the nar- 
rowing bay. 

If we observe the 
variation in height of 
the tide at any single place, we find that its change cor- 
responds with the variations in the phases of the moon, 
and this would lead us to believe that in some way the 
tide is caused by the moon. 

The moon, the nearest of the heavenly bodies, is at all 
times exerting a pull upon the earth, just as this is upon 




Fig. 101. 

Diagram to show advance of tidal wave in 
the Atlantic. Figures represent hours of 
the day; heavy lines, noon. 



© 







212 FIB ST BOOK OF PHYSICAL GEOGRAPHY 

the moon. This pull is the attraction of gravitation, by 
which the bodies of the solar system are bound together, 
and it is something like that which holds the air to the 
earth and causes a stone to fall to the ground. If the 
moon and earth could cease revolving, they would come 
together as certainly as a stone will fall to the earth if it 

is dropped from 

the hand. There- 

/^ fore the moon is 

® endeavoring to 

draw the earth 

U toward it, and to 

B a slight extent is 

^ really succeeding 

in drawing the 
^ / ^ liquid part of the 

earth. Therefore 
a wave is raised 
in the ocean, and 

Three diagrams to show (A) the sun, earth, and ^^ ^^® moon pass- 
moon in line at new moon; (B) the same at full es through the 
moon ; and (C) the moon and sun in opposition -, a.-\ j.- j 

during the quarter. In the first two cases the neavens, tlie tlCle 
waves of moon and sun are formed in about the wave f ollows it. 
same place ; hut in the third they are formed in ^ , . 

different places and hence the tidal rise is less. -^y tillS attrac- 

tion one wave is 
formed under the moon, and one on the opposite side; 
but they do not pass around the earth directly under the 
moon, for they lag behind, and hence follow it. 

What the moon is doing in this respect, the sun is also 
doing ; but this body, though much larger than the moon, 
is so much further from us that its influence is less, 
because the attractive force varies not only with the mass 




© 



Fig. 102. 



THE MOVEMENTS OF THE OCEAN 213 

of the body, but also with its distance. Therefore four 
waves are formed, the two larger ones by the moon, and 
two much smaller waves by the sun. At new and full 
moon, the sun, earth, and moon are nearly in line, and 
then both the lunar and solar tides are raised in about the 
same place. Then the two combine to form a larger 
wave than usual, giving the spring tide. When the moon 
is in the quarter, the sun and moon are exerting their 
influence in directions nearly at right angles to one an- 
other, and the two waves are therefore somewhat in oppo- 
sition, so that a lower or neap tide is caused. Therefore, 
as the phases of the moon change, the height of the tide 
varies. 

There are other causes for variations, one of the most 
important being the difference in distance of the moon. 
This body travels around the earth once in a lunar month, 
not in a circle, but along an elliptical path with the earth 
at one of the foci. Therefore at one part of the lunar 
month, the moon is much nearer than at the other times. 
When nearest to us the moon is said to be in perigee^ and 
when furthest in apogee. The attraction varies greatly 
with the distance of the body ; and hence when the moon 
is in perigee, the tide is made high, particularly if the 
moon is full or new.^ 

Effects of the Tides. — Sometimes the tides, instead of being merely 
the rising and falling of water in true wave form, become real cur- 
rents. Then navigation is checked or aided, sand and clay are drifted 
about, bars are formed at the mouths of harbors, and the waves are 

1 An especially valuable lesson in tide variation may be given by a 
careful study and plotting of the tide height at various places, as given in 
the Tide Tables for the Atlantic, published by the United States Coast 
Survey, Washington, D.C. ; price 25 cents. 



214 FIRST BOOK OF PHYSICAL GEOGRAPHY 

aided in moving the sediment either along the coast or else out to sea. 
These currents are caused by the approach of the wave over the shal- 
lovi^ bottom, in a manner similar to the approach of the wind-wave 
surf on the beach. As the tide rises, the current moves one way, and 
the outgoing tide in the opposite direction. Sometimes these tidal 
currents are so strong that navigation is impeded, and oftentimes it 
is impossible to row a boat against the current. In parts of the Bay 
of Fundy, where the tide rises to such great height, the currents or 
j^aces become as violent as a rapid river current, and it is unsafe to 
attempt to navigate these waters unless one is perfectly familiar with 
all the peculiarities of movement. There the tide, advancing over 
the low mud flats, moves as rapidly as a man can run. 

There are many special causes for these currents. Sometimes the 
tide rises higher on one side of a peninsula than on the other, and 
then, if there is a strait between the two bays, with the rising and 
falling tide the water moves backward and forward through it. 
Again, after passing through a narrow arm of the sea, the water may 
enter a broad bay ; and then, when the tide falls, this great mass of 
water rushes out through the narrow channel with the velocity of a 
river. On the other hand, the tide may sometimes lose its height and 
velocity, when after passing through a narrow entrance it reaches 
a broad sea, as the tide wave does after passing through the Strait 
of Gibraltar into the Mediterranean, where there is almost no rise 
and fall of the tide.^ 

This silent and regular rising and falling of the ocean 
surface is one of the most interesting features connected 
with this great expanse of water. On the open sea one 
might travel for thousands of miles, never knowing of 
its existence ; but along the land margin it becomes very 
apparent and important. 

1 Here, however, a separate tide of small size is generated by the same 
cause which makes the great ocean tidal waves. In fact, even in large 
lakes there are slight tides of this origin, besides the more irregular fluct- 
uations of the surface due to winds and changes in the air pressure, and 
known under the name of seiches. 




Facing page 21k. 



Diagrammati( ha 




c ocean currents. 



THE MOVEMENTS OF THE OCEAN 215 

Ocean Currents : Differences iyi Temperature — When 
speaking of the temperature of the sea (Chap. XII), it was 
stated that there are reasons for believing that there is a 
circulation in the ocean, consisting of sinking water in 
the colder latitudes, and rising in warmer belts, with bot- 
tom currents moving toward the Equator, and surface 
movements away from it. This conclusion seems neces- 
sary as a result of the fact of greater warmth in the one 
place than in the other, and there are other facts pointing 
to the same conclusion. The temperature of the deep sea 
can be accounted for only on this explanation. Moreover, 
if there were no such circulation, how could the deep-sea 
animals obtain the oxygen which they need ? If the waters 
were quiet, these creatures would have no source for this 
necessary element; but a slow circulation along the bot- 
tom, supplied from the surface, would furnish it. 

On this theory the ocean is somewhat like the air, and 
the great movements of ocean currents are similar to the 
circulation of the atmosphere ; but there is one very 
important difference : the air is warmed from below, and 
being made lighter near the ground, rises in the warmer 
belts, while it cools and settles in the colder regions. But 
the sun's heat does not reach to the bottom of the sea, and 
the warming of this is therefore confined to the surface 
layers ; hence the comparison with the air is not strictly 
correct. There are actual movements of the ocean similar 
to those which this theory demands, but they seem 1j be 
more powerful than this cause alone could produce ; and 
although nearly every one believes that differences in tem- 
perature cause a slow circulation of cold water along the 
ocean bottom, and aid in the production of some of the cur- 
rents at the surface, another explanation seems to be neces- 



216 FIBST BOOK OF PHYSICAL GEOGRAPHY 

sary for the more powerful of the surface currents. Before 
stating this theory let us look briefly at the actual condi- 
tions of oceanic circulation. 

Atlantic Currents. — For illustration of these ocean move- 
ments we may select the north Atlantic, in which the 
ocean currents are as well developed as anywhere, and 
much more carefully studied. ^ There is a slow circula- 
tion of the sea, both north and south of the Equator, mov- 
ing toward the belt of calms in the direction followed by 
the trades, — that is, southwest on the northern side of the 
Equator, and northwest to the south of it. In the dol- 
drums, between the trade-wind belts, this drift of water 
moves westward until the coast of South America is 
reached, where it divides into two unequal parts, the 
larger journeying northward along the northern coast of 
South America, the smaller southwards. The former 
passes as a slow drift of water, partly into the Caribbean, 
partly between the West Indies, and partly outside of 
these islands in the open sea. The latter, turning to the 
right under the influence of the earth's rotation, circles 
eastward, then southeastward, and finally, passing along 
the coast of Europe and northern Africa, again comes within 
the zone of the trade winds. Hence it circles around, 
forming a great eddy of slowly moving surface water, 
and this is found in all the oceans that are crossed by the 
Equatorial belt, turning to the left in the southern seas 
and to the right in the northern. 

That part of the Equatorial drift which passes into the 
Caribbean Sea, circles through it, becoming warmer, enters 

1 Although some of the conditions on the ocean bottom have been 
investigated, it has not been found possible to determine the rate of move^ 
ment of the cold bottom water. 



THE MOVEMENTS OF THE OCEAN 



217 



the Gulf of Mexico, and passes out of it between the end 
of Florida and Cuba, where it emerges as the Gulf Stream 
(Fig. 103). This current, flowing rapidly at first, as a 
narrow stream, loses velocity as it passes along and becomes 
broader. Off the Florida coast it is a distinct stream in 
the sea, flowing at the rate of four or five miles an hour ; 
but by the time the 
latitude of Cape 
Cod has been 
reached, its veloc- 
ity is reduced to 
less than two miles 
an hour. For a 
while it flows near 
the American 
coast, then, about 
in the latitude of 
Cape Hatteras, it 
slowly turns to the 
right, leaving the 
American shores 
and crossing the 
Atlantic to Europe, 
being deflected in 
this direction by 
the earth's rota- 
tion. Against the 
European coast it divides, some turning southwards toward 
the Equator, and some going northward past Scandinavia 
into the Arctic.^ 

1 Nansen has proved that there is a current in the Arctic from the 
northern shores of Asia to the region between Greenland and Spitzbergen. 




Fig. 103. 

Diagram to show the currents of the eastern 
north Atlantic. Figures tell rate of movement 
in miles per hour. 



218 FIBST BOOK OF PHYSICAL GEOGBAPHT 

In the north Atlantic there is a cold current called the 
Labrador current, flowing from the Arctic. This comes 
down from the north, between Greenland and Baffin Land, 
past Labrador, and as far as New England, where it dis- 
appears in the Gulf of Maine.^ It hugs the American 
coast closely, being turned to the right by the deflective 
effect of the earth's rotation. 

Aside from similar currents in the several oceans, there 
is a drift of water in the southern hemisphere, south of 
Africa, South America, and Australia, where the water 
moves in the direction followed by the prevailing wester- 
lies. These movements constitute the great ocean cur- 
rents of the globe, and as a result of them the waters of 
the sea are almost constantly in motion in certain definite 
directions. 

The Explanation. — It will be noticed that where they 
start, the currents have the same directions as the pre- 
vailing winds,^ and this is believed by many to be the 
most prominent cause for the surface ocean currents. 
Water is always drifted before the wind, and small cur- 
rents of this origin may be seen along the coast, not only 
in the ocean but in many lakes. Drifted along, turned by 
the land, and by the effect of rotation, the water circles 
through the seas, giving the great oceanic circulation. 

No doubt there is also a great but slow movement 
caused by differences in temperature ; but this seems to be 
a less important cause for surface currents than the winds. 

1 The gulf enclosed between Nova Scotia and Cape Cod. 

2 The Labrador current is possibly started by the north winds, but 
probably is a partial return of the water that is sent into the Arctic by the 
Gulf Stream, and the fresh water that enters the Arctic from the great 
north-flowing rivers of Asia and North America. Even the Gulf Stream 
is aided in crossing the Atlantic by the prevailing westerlies. 



THE MOVEMENTS OF THE OCEAN 219 

There are several reasons for this conclusion, but the most 
prominent one is, that the ocean currents are less pro- 
nounced in the southern than in the northern oceans ; yet 
southern oceans are open to the cold Antarctic, while 
those north of the Equator are more nearly closed to the 
cold Arctic waters ; and particularly is this true of the 
north Pacific. If the oceanic circulation is chiefly due to 
differences in temperature, it should be most pronounced 
in the southern oceans, where there is a greater chance for 
this difference to express itself ; but the reverse is true. 

Effects. — Ocean currents are chiefly important in modi- 
fying the climate. By them the ocean itself, and the 
neighboring lands, are made either Avarmer or cooler. The 
currents do not cut the shores as do the waves, nor are 
they moving rapidly enough to carry much sediment, as 
are some of the tidal currents ; but they are performing 
a great work in nourishing large numbers of marine ani- 
mals, which float in these waters, and which furnish food to 
the larger animals. The warm ocean currents are food 
bringers for the colonies of corals which exist in the 
warmer portions of the ocean, and they therefore aid in 
the building of many lands in the sea. The Bahamas, 
the Bermudas, the southern end of Florida, and the multi- 
tudes of islands in the south Pacific, are coral islands 
made from the skeletons of creatures nourished in the 
warm water of the ocean currents. 



PAET lY. — THE LAND 



CHAPTER XIV 

THE EARTH'S CRUST 



Condition of the Crust. ^ — Nearly everywhere at the 
very surface of the land there is a soil covering, beneath 
which, at depths usually not more than a few feet, though 




Fig. 104. 
A rock made of horizontal layers of different kinds. 

sometimes 200, 300, or even 400 feet, there is solid rock, 
and this continues to as great a depth as man has pene- 

1 At this point I would suggest a review of a part of Chapter I, particu- 
larly pages 7-13. 

220 



THE earth's crust 221 

trated into the crust. These rocks are of many different 
kinds, and when seen in a river gorge, or any other cut- 
ting, they are usually found to be in layers, and in many 
cases these are arranged one on another, generally in hori- 
zontal beds (Figs. 104, 136, and 139), but sometimes in 
layers tilted at various angles (Chapter XIX), perhaps 
even vertically. Some of these are made up of fragments, 
such as grains of clay or sand, some of shells of animals, 
forming limestones, and some are made of distinct min- 
erals, and these are called crystalline rocks. 

Minerals of the Crust : ^ Elements. — When subjected to 
chemical analysis, it is found that in any rock there are 
certain elements.^ Chemists have so far discovered about 
70 elements, but only a very few are really common 
in the earth, the three most abundant being oxygen, sili- 
con, and aluminum; and the others, that are less abundant, 
though still important in the crust, are iron, calcium, 
magnesium, potassium, sodium, carbon, and hydrogen. 
Oxygen forms 47% of the earth's crust, silicon 27%, and 
aluminum 8%, so that these three constitute about 82% of 
the known rocks of the earth's crust. In many of the 

1 No attempt can be made here to treat the subject of mineralogy. 
A very good elementary book is Dana's Minerals and How to Study 
Them ; and in his larger works will be found a more complete treatment. 
The only way for students to really understand minerals is to examine 
them and study their characteristics from actual specimens. I would 
suggest the introduction of some simple laboratory work in the study of 
minerals, preferably taking up only a dozen or twenty of the most com- 
mon, which are selected not so much for crystal perfection as to show the 
other characteristics. 

2 An element is the simplest form to which man has been able to reduce 
matter. Gold, for instance, cannot be made simpler, nor can mercury, 
nor the oxygen of the air ; but each of these may be made to combine 
with other elements to form some chemical combination. 



222 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



rocks, such as those made of clay, the elements are often 
in complex and indefinite combination ; but frequently they 
are present in distinct combinations, known as minerals. 

Definition. — A mineral may be defined as a homogeneous 
solid of definite chemical composition^ occurring in nature^ hut 
not of apparent organic origin. There are perhaps 2000 
minerals known, but most of them are so rare that they 
are found only in the larger mineral collections. One 
or two hundred may be considered fairly common, but 
less than a dozen are really important in the rocks. 

Quartz. — • By far the most abundant of these is quartz^ 
a mineral composed of the two elements silicon and oxygen 

(SiOg), and found in all 
granites and sandstones, 
as well as in many other 
rocks and most soils. It 
is so hard that it cannot 
be scratched with a knife, 
and it will cut glass. 
Very often it is found in 
perfect crystalline form, 
being bounded by smooth 
glassy planes, which form 
a six-sided prism, while on one end, or perhaps on both, 
there are hexagonal pyramids. More commonly the quartz 
crystal is less perfect, and in fact most of the quartz of 
the world occurs in grains, generally of small size. 

Quartz may be told from the single grains of other com- 
mon minerals by its hardness, and by its glassy surface, 
which reflects light as glass does ; and for this reason it is 
said to have a glassy lustre. It resembles glass also in its 
fracture, for when it breaks it has a shelly, rounded sur- 




FiG. 105. 

Quartz crystal, showing three of the prism 
sides and three of the pyramid faces. 



TBB EAUTB'S CRUST 223 

face quite like broken glass. This is called the conchoidal 
or shells/ fracture. Being hard, quartz tends to make 
rocks durable, for it is not easily ground down by the 
agents which are cutting rocks (Chapter XV). It is 
more durable than other minerals for another reason, — 
when exposed to the air it does not change and crumble, 
as some minerals do, but throughout all the attacks of the 
weather remains pure, clear quartz. Quartz, when com- 
pared with the mineral next described, is about as gold is 
to iron. In the air gold remains fresh, but iron soon rusts 
and becomes soft. Quartz is also slightly soluble, and the 
water flowing over the land always carries small quanti- 
ties of it ; but in this respect it is much less soluble than 
calcite, and more so than feldspar. Therefore in most 
respects this is a very durable mineral. 

Feldspar. — This mineral is another exceedingly impor- 
tant component of the crust. With quartz it occurs in 
granite, and this rock is chiefly made of these two miner- 
als. Feldspar sometimes occurs in crystals, though much 
more rarely than quartz. When it is found in grains 
together with quartz, the two can generally be told apart 
by the fact that feldspar is duller and Avhitish, and has a 
much less glassy lustre. Besides this, a fractured piece 
is found to be smooth, like the side of a crystal. This is 
because the mineral is cut by many minute planes, extend- 
ing parallel to one another, which are known as cleavage 
planes (Fig. 106) ; and when the mineral is broken, it 
splits most easily along these planes. Therefore, al- 
though nearly of the same hardness (quartz being a 
little harder), these minerals may be easily told apart.^ 

1 Excellent observation lessons may be given by means of small chips 
or pebbles from a stone yard or stream bed. 



224 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



Although nearly as hard as quartz, and not a soluble 
mineral, feldspar is not nearly so durable. This is because 
when exposed to the air it slowly changes and gradually 
crumbles. Really fresh feldspar is glassy and looks quite 
like quartz; but this kind of feldspar is not common 
excepting in recently cooled lavas, in which there has not 
been time enough for change. The dull opaqueness of 
most feldspar is due to the beginning of this change ; and 
when it has gone far enough, the formerly hard mineral is 
changed to a crumbling clay or kaolin^ just as hard iron 
or steel may rust to a soft, yellow, clayey mass. Air and 

water cause many changes 
among the minerals, and hence 
they slowly crumble when ex- 
posed to the action of these 
agents. Since feldspar is so 
easily altered, it is an element 
of weakness in any rock. 

Calcite, another common 
mineral, forms the marbles 
and limestones, and is present 
in small quantities in many 
other rocks. It illustrates 
still other mineral properties. 
Like feldspar it has cleavage, 
but this is much more distinct in calcite, and one can 
rarely find a piece of it in which the smooth, shiny cleav- 
age faces do not appear. ^ In color it varies greatly, as 
do the minerals described above. While quartz and feld- 
spar cannot be scratched with a knife, calcite is easily 
cut. Moreover, unlike these two minerals, it is readily 
^ Por instance, in the white marble used for monuments in cemeteries. 




Fig. 106. 

A piece of calcite, showing cleav 
age faces and cleavage cracks. 



TSE earth's crust 225 

carried in solution, and the water of the land and sea 
always carries it.^ One may see th6 proof of this by 
visiting a cemetery in which the stones have stood for 
some years. The surface of the marble headstones is 
often etched by the rain, and sometimes the inscriptions 
are nearly obscured. 

When exposed to the action of the air, calcite does not 
change or decay, though it may be carried off in solution ; 
but if water bearing some acid comes in contact with it, 
a chemical change immediately takes place, and the calcite 
slowly disappears. What happens then may be seen on 
a very much more extensive scale, by placing a bit of 
calcite or marble in a weak solution of hydrochloric, 
or other acid, when an effervescence begins and carbonic 
acid gas escapes, until finally the calcite is entirely gone. 
Because this mineral is soft, easily dissolved, and changed 
when acids come in contact with it, rocks that are made 
of calcite are not nearly so durable as those made of quartz 
or feldspar, or of these combined. 

There are numerous other important mmerals in the rocks, but the 
three above mentioned, together with the clays, etc., which have been 
formed from their destruction, constitute considerably more than one- 
half of the earth's crust. The other more common rock-forming min- 
erals are the micas, some of which are colorless and others black, while 
all are characterized by a remarkable cleavage ; hornblende and augite, 
dark brown, green, or black minerals; and some of the comj)ounds of 
iron, which give the red and yellow colors to soils and many rocks. ^ 

1 It is this which makes it possible for animals to build shells. 

2 The study of these and a few other minerals, in which each student 
is expected to handle the specimens and determine the lustre, cleavage 
(if present), fracture, color, hardness, crystal form (if present), specific 
gravity, etc. , will be of great disciplinary value, particularly if in study- 
ing mineral specimens, some of the crystalline rocks are furnished them 
to study and to identify the minerals of which they are made. 

Q 



226 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



Rocks of the Crust : Igneous Rocks. — Rocks like those 
found on the land are even now forming on the earth's 
surface. Whenever a volcano breaks forth in eruption, 
and lava flows out at the surface, a new rock is being 
added to the crust, and at all times in the past, similar 
lavas have come from within the earth, and upon cooling 
have formed hard rocks (Chapter XX). Upon the flanks 

of Vesuvius and other volcanoes, 
we find such lavas which have 
recently cooled; and in former 
times, volcanoes existed and sent 
forth molten rock in parts of the 
earth which are no longer the 
seats of eruption. 

If we examine a solidified lava, 
either one that has just cooled, 
or one that was formed long ago, 
we find that it is made of min- 
erals, though the mineral grains 
may be so minute that none can 
be distinguished by the eye. 
(Compare Plate 17 (diabase) and 
Fig. 107.) The minerals that are 
most common in these beds are 
feldspar, quartz, hornblende and 
augite, and the grains vary greatly 
in size. When the lavas are melted, they are made of 
elements which are not definitely combined; but as they 
cool, and begin to form hard rock, these elements come 
together and form definite compounds, or minerals, some- 
what as salt crystals are produced when a solution of hot 
salt water is cooled. If the lava cools very slowly, the 




Fig. 107. 

Section of diabase (Plate 17) 
enlarged under the micro- 
scope, showing the minerals. 




i 



TBE earth's crust 



227 



^O^CAJ\/q 



crystals have time to grow to large size (Plate 17) ; but if 
the cooling is rapid, they may be so small that the un- 
aided eye cannot distinguish them ; and indeed the rock 
may even become a glass, like obsidian, in which even the 
microscope cannot detect crystal grains. 

Some lavas which are thrust into the earth, and which 
have not reached the surface, have cooled so very slowly 
that the mineral grains have all grown to good size. This 
is the origin of the granites (Fig. 108) ; and such a rock is 
seen at the sur- 
face only when 
the layers be- 
neath which it 
was formerly 
buried have 
been removed 
by the agents 
that are always 
at work wear- 
iug away the 
crust (Chapter XV). Not only are there these differ- 
ences in texture^ but some lavas are porous, others dense, 
and in fact some are so porous, that like pumice they will 
float on water. The pores are caused by the expansion 
of steam in the cooling lava, for all of these molten rocks 
contain water. There is also a difference in chemical com- 
position of the lavas, and hence in the kind of minerals 
which grow as they cool. As a result of this, some rocks 
contain quartz and others have none of this mineral, some 
have hornblende, some augite, etc. As these differences 
occur there is a variation in the color, some being black 
and some nearly white. In accordance with these differ- 




FiG. 108. 

Diagram to illustrate the way in which granite is 
thrust into the earth and later reached. 



228 FIRST BOOK OF PHYSICAL GEOGRAPHY 

ences the igneous rocks (those formed from the cooling of 
lava) are classified and given different names.^ 

Sedimentary/ Bocks. — When exposed to the air, lavas 
and all other rocks are subjected to changes, resulting from 
the decay of some of the minerals (like feldspar, augite, 
and hornblende), the solution of others, and the breaking 
up of the grains by action of frost, etc. (Chapter XV), 
so that in the course of time the rocks slowly crumble. 
As a result of this, there are quite different products of 
the changes in such minerals as feldsjjar. New chemical 
compounds are produced, some being soluble and others 
insoluble. The former may pass off in the water, which 
is always sinking into the ground, but the latter remain 
behind, generally in the form of fine-grained clay, like 
kaolin. A third product is the unchanged mineral, like 
the grains of quartz. All of these, taken by the water 
which flows over the land, find their way into the rivers, 
then from these either into lakes or the sea. Here the 
waves take the fragments, adding other substances which 
they themselves rasp from the coast, and strew them 
over the bed of the sea near the land. The beds of rock 
thus formed are called sedimentary/, and upon the land 
there are great areas of these, which were once formed 
in the sea, but are now raised above it. 

The soluble substances, like salt, gypsum, or carbonate 
of lime, may in some rare cases, as for instance in salt 

1 It is impossible to introduce into this book a more complete state- 
ment concerning these rocks ; in fact it would not be profitable to do so 
unless the student were supplied with specimens for individual study. 
This form of laboratory work I would strongly urge. A more complete 
statement about the rocks of the crust is contained in my Elementary 
Geology^ where the teacher will find the necessary information concern- 
ing kinds of rocks, names, places where specimens may be purchased, etc. 



THE EARTH'S CRUST 



229 




lakes, be precipitated from a saturated solution, forming 

layers of chemically/ deposited rock. There are salt and 
gypsum beds in the west which have been formed in 
this way;^ and in some of the salt lakes of the Great 
Basin of the west, beds of carbonate of lime are even now 
being precipitated. Or the carbonate of lime may be 
taken from the water by animals and built into their 
shells, which later gather 
into layers of carbonate of 
lime, forming limestone. 
The Globigerina ooze and 
the beds of coral lime- 
stone, which are accumu- 
lating near coral reefs, are 
instances of these rocks, 
which are known as organic 
sedimentary beds. Coal is 
another illustration of this 
class ; but in this case the 
beds are made of plant frag- 
ments which have taken 

substances from the air, as well as from the water of the 
ground. In every tree there are mineral substances, and 
it is these which form part of the wood and coal ash. 

But by far the most important group of sedimentary 
rocks is the mechajiical or fragmental^ which are made of 
rock fragments of all kinds, which the waves and currents 
have carried and deposited in layers or strata on the sea 
bed. These fragments vary in size from the very finest 
clay to coarse pebbles and even boulders. Sometimes, 
when the waves are very strong, the latter may be carried; 
1 This is the origin of many of the beds of rock salt which are now mined. 



Fig. 109. 

A pebble bed, a part of a beach formed 
in the coal or Carboniferous period, 
now exposed at Cape Breton Island, 
Nova Scotia. 



230 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



but later, when the sea is more quiet, only the smaller 
fragments can be transported. Near the coast line, where 
the waves are violent, the deposits are of coarse pebbles 
or sand (Fig. 110) ; but in quiet bays, and far out to sea, 
the finer clay, which can no longer be kept afloat by the cur- 
rents, settles to the bottom and forms beds of clay. There- 
fore among these 
rocks there is a 
variation in tex- 
ture from peb- 
ble beds to clay 
rocks; and the 
members of this 
group are named 
upon this basis, 
the pebble rocks 
being called con- 
glomerates (Fig. 
109), the sand 
beds sandstones^ 
and the clay lay- 
ers either clay- 
stones or shales. 
Like the lavas, 

the fragmental rocks are unconsolidated when first formed ; 
but as they are not hot they do not become solid by cool- 
ing. When found upon land these strata are generally 
in the form of hard beds ; and by examination it will be 
found that the grains of which they are composed are held 
together by a cement^ somewhat as we may cause sand 
grains to cling together by means of mucilage. The 
cement of these rocks is deposited from solution by the 




Fig. 110. 
A sand beach, pebbly at one end, Cape Ann, Mass. 



THE EARTH'S CRUST 



231 



water which is percolating between the grains. The most 
common rock cements are silica, carbonate of lime, or some 
salt of iron, ^substances which are being carried in solu- 
tion by most of the water which is flowing over the sur- 
face of the earth, and sinking into the ground. Sometimes 
the cementing has only gone far enough to cause the 
grains to adhere very slightly, but more often a hard and 
very dense rock is formed by means of this cement. This 




Fig. 111. 
A specimen ol coquina. 

process of cementing may often be seen in gravel beds and 
on shell beaches, like those of Florida, where the shell 
fragments which are thrown up by the waves, soon become 
transformed into a rock called coquin (Fig. Ill), which 
is used for building houses in these regions. Betw^een the 
clay or sand bed and the solid rock there is every gradation. 

Metamorphic Rocks. — Igneous rocks are made of minerals which 
are fine crystals, and always have a crystalline structure, though 



232 FIBST BOOK OF PHYSICAL GEOGBAPHT 

sometimes not possessing a perfect crystal outline. The chemically 
deposited sedimentary rocks are also frequently crystalline ; but the 
sedimentary beds proper, when first formed, are made up oi fragments 
of minerals, and are not crystalline. However, the rocks of the crust 
of the earth are subjected to many changes. The water passing 
through them alters them so that beds of shell may become crystal- 
line calcite ; or the heat of a lava mass passing through the rocks, or 
the heat which exists in the earth, may also cause change. Besides 
this, among mountains the strata are often folded, and sometimes even 
crumpled (Fig. 113), as we might crumple sheets of paper; and this 
also causes heat and change. The heat results from the friction of 
the rock particles as they glide over one another during the folding, 
as we may warm two rocks by rubbing them together, or by pressing 
them against a grindstone that is revolving. 

From these various causes a third great class of rock beds is formed, 
to which the name metamorphic is given. These resemble the igneous 
in being crystalline, and some of them look quite like granite. Here 
however, the minerals have been formed not by melting, but by some 
change, or metamorphism, in which heat and heated water have 
caused minerals to change in kind and form. The limestone becomes 
a marble, the clay rock a slate, and perhaps even a schist; and meta- 
morphism may produce a coarse-grained granitic rock, called gneiss 
(Plate 17). These metamorphic beds are nmch more common in 
mountainous regions than elsewhere, because here the cause for 
heat has been more pronounced. They exist over great areas in 
Canada, ISTew England, the highlands of ^N'ew Jersey, etc. 

Position of the Rocks. — Lavas may exist in any position, 
but they are commonly found either in nearly horizontal 
beds or in steeply inclined sheets (Figs. 108 and 112). 
The reason for this is evident; for they come from deep 
within the earth, and cut through the rocks nearly verti- 
cally in reaching toward the surface, where if they come 
to the air, they flow out over the land in nearly horizontal 
sheets, as any pasty liquid would. These may later be 
buried beneath other rocks, or the wearing away of the 
crust may reach down to a buried lava mass. The sheets 



THE EARTH'S CRUST 



233 



,OUCA/v, 



^0. 






Fig. 112. 
Diagram showing a volcano in cross section. 



that cut through the strata are called dikes (Figs. 108 and 
112) ; and in every eruption of Kilauea, in the Hawaiian 
Islands, dikes are formed 
on the flanks of the vol- 
cano, as the lava wells 
out from a fissure and 
flows down the mountain 
sides (Chapter XX). 

In the sedimentary 
rocks the Oidginal condi- 
tion is nearly horizontal 
beds of different kinds. 
Great sheets of sand or 
clay are formed over the 
bed of the sea, and the grains settle to the bottom, form- 
ing layers parallel to it, which are nearly horizontal, 

because this is the out- 
line of the greater part of 
the ocean floor (Chapter 
XII). The layers vary in 
kind and in texture, for 
as time elapses there are 
many changes. The cur- 
rents vary, the velocity 
of the waves changes, 
and the very level of the 
land fluctuates. These 
changes in conditions 
may cause the deposit first of a sheet of clay, then of 
sand, then another of clay, then one of limestone, etc. ; 
and these layers may be thin seams (or laminoe) or great 
beds. These variations in kind of rock produce stratifica- 




FiG. 113. 
Crumpling of rock. 



234 FinST BOOK OF PHTSiCAL GFOGRAPBT ¥ 

tion, and the different beds are called strata (Figs. 104, 
136, and 139). This difference of stratification is one of 
the most characteristic features of the sedimentary rocks, 
which in consequence are often called stratified. When 
raised into a dry-land condition, these beds are most 
commonly so uplifted that they still remain nearly hori- 
zontal, as in the great Mississippi valley between the 
Appalachian and Rocky Mountains ; but sometimes they 
are folded, as in the case of these mountains (Chap. XIX). 

Metamorphic rocks also vary in position, for very often they are 
altered beds which were originally stratified into layers of different tex- 
ture and composition ; but they are rarely horizontal, for they have been 
metamorphosed as the result of changes in which folding has been of 
importance. Indeed, the metamorphic rocks are usually most com- 
plexly bent and twisted (Fig. 113) ; for when, by the movement of 
the earth's crust, these contortions of the rocks occur, they are so 
folded and changed that they are no longer sedimentary or igneous, 
but become members of the metamorphic group. M 

Movements of the Crust. — The study of the rocks proves 
that the earth's crust has been in movement in the past. 
On the land, even on the tops of mountains, there are ' 
strata which are the same in kind as those now gathering | 
on the sea floor ; and in them are found entombed the shells i 
or skeletons of animals that once lived in the sea. Hence I 
these rocks tell us that the land where they now exist was 
once a sea bed, and that then there came an uplift, as the 
result of which they have been raised perhaps as much 
as 10,000 feet above sea level. Sometimes this uplift has j 
been such as to leave the rocks still in horizontal sheets ; 
but in other places the strata have been folded and broken, 
especially among mountains (Chap. XIX). 

Not only has the crust of the earth been in movement 



THE EARTH'S CBUST 235 

in the past, but it is even now changing position. During 
the earthquake shocks in 1835 the land on the coast of 
Chile was lifted four or five feet, and during many earth- 
quakes similar movements have occurred in other places. 
The shores of Baffin Land have risen so recently that 
pebble beaches formed by the waves, and now lifted above 
the sea level, have not been exposed to the air long enough 
to have been covered with moss and lichen growth. There 
is historical proof of change in level (rising) of the coast 
of Hudson's Bay; and on the shores of New England and 



r I I i i I 



I I I 1^ 




!Lli I I ! __!_ _^ l^ ^^r?^"^^^^^^ 



Fig. 114. 

A section of horizontal rocks showing three fault planes. 

New Jersey, tree trunks now standing below sea level 
show a recent sinking. On the coast of Sweden part of 
the land is slowly rising and part sinking, as has been 
proved by careful measurements made under the super- 
vision of the government. Scores of similar instances 
might be introduced to show that the crust is rising here 
and sinking there ; and from equally conclusive evidence 
geologists have proved that in the past, these changes, 
occupying long periods of time, have produced not only 
the mountains of the land, but even the continents. 

When the rocks thus moved are disturbed very much 
from their horizontal position, they either bend or break. 
When breaking, they may move only a few inches, or per- 
haps thousands of feet, along the plane of breaking, ot fault 



236 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



plane (Figs. 114 and 115). Faults are very common 
among mountains, and in fact some mountainous eleva- 
tions are caused by them, the broken blocks being raised 

and tilted. In folding there 
may be an up or a down fold 
(Fig. 116), the former being 
called an anticline, the latter 
a syncline ; and while these 
are sometimes very symmet- 
rical, they are often irregular, 
and sometimes they are even 
in the form of overturned 
folds. In fact, the folding 
of rocks may proceed so far, 
that as among the metamor- 
phic series, the beds are act- 
ually crumpled (Fig. 113). 
In an anticline the rocks dip 
in two directions away from 
a central axis; but there is a fold, the monocline (Fig. 
117), in which the strata dip in only one direction. 

Age of the Earth. — Various attempts have been made 
to state the age of the earth in years ; but all have been 

OS OA 




Fig. 115. 

Photograph of a small fault near 
Sydney, Cape Breton. 




Fig. 116. 
Section of folded rocks showing anticlines (A), synclines (S), unsymmetrical 
anticlines and synclines (U.A. and U.S.) and overturned anticlines and 
synclines (O.A. and O.S.). 

far from the truth, because it is impossible to find any 
means of telling how long it takes for Nature to perform 



THE earth's cnusT 237 

her tasks in changing the earth's surface. In some places 
there are 30, 000 or 40, 000 feet of sedimentary strata, 
one layer upon another, that have been deposited in the 
sea during some past time ; and we know that the action 
of the agents of the sea would require many scores of 
thousands of years to form these miles of rock. 

Some volcanoes have been watched for one or two thou- 
sand years, and they have not grown much in size ; yet in 
previous times they have been built to very nearly their 
present great height, which in some cases is a mile or two 
above the base. Not only this, but in some parts of the 



72222 


•>^-^ 






^r^ 












= ^^- 


--■^r._ 


= ^^---- 


1^-/5:^;:^^^^ 




:••.':•.■••••• 


~~Z 




2^ 


l"-^' 


1=3^ 


:^^? 


^^ 


,■•'■■•'.■ 






•^\r>v:v. 


-:s:<-:x 


^^ 




_-^-^- 


_-<i. 


^fe^-?^- 


§ 


J 


'II 




^^- 





Fig. 117. 
A monocline. 

earth great volcanoes have been built, then have become 
extinct, and then slowly been destroyed, until now only a 
few remnants of the lofty pile of lava remain to tell the 
story. Such great changes require much time; and so 
also does the formation of the deep river valleys, like the 
Colorado, which in a lifetime do not appear to change, 
yet which have been cut out of the rocks by the slow 
action of the river water. 

The difficulty in attempting any estimate of the age of 
the earth comes from the fact that the changes are very 
slow, and the time since they began to operate very great, 
while our lifetime is short. Compared with the time 
which has elapsed since the beginning of the geological 
history, a human life is but a fraction of a second. One 



238 FIBST BOOK OF PHYSICAL GEOGRAPHY 

thing is noteworthy: all who have tried to estimate the 
age of the earth during recent years have placed it as 
millions of years^ and some, hundreds of millions. 

Geological Ages. — While we cannot tell the age of the 
earth in years^ we have been able to work out its general 
history, and to divide this into stages ov periods, just as 
we divide the early history of man into stages (the paleo- 
lithic and neolithic), before he began to leave written 
records, which can be used to tell the actual time. These 
geological stages have been made out from the records left 
in the rocks by the animal life of the past. Slowly, 
throughout all past time, animals and plants have been 
changing, being first of simple and lowly forms, and 
gradually changing as higher groups appeared. For 
instance, at first there were no true fishes, reptiles, birds, 
or mammals ; then fishes appeared, and then reptiles, then 
birds, then mammals, and finally, highest of all, man him- 
self. Careful studies have revealed this history of change, 
and we are now able to divide the earth historj^ into stages 
or ayes, and say that certain rocks, in which are found 
animal and plant remains of certain kinds, belong to an 
earlier or later period than other beds in which different 
organic remains, or fossils, occur. 

To these divisions of the history certain names have 
been given, and we have a kind of rough chronology, or 
time scale. In the table, the most ancient periods are 
placed at the bottom. These are merely the names for 
the larger divisions ; but geologists have carried the chro- 
nology much further, and there are many names represent- 
ing subdivisions of these larger groups.^ 

1 A more complete statement of the basis for this chronology will be 
found in most geologies. 



THE earth's crust 239 

TABLE OF GEOLOGICAIi AGES 



CEN020IC 
TIME. 

Age of 
Mammals. 


Pleistocene 

or 
Quaternary. 


Man assumes importance, particularly 
in the upper part. In the first half the 
Glacial Period prevailed. 


Neocene. 


1 


Mammals develop in remarkable vari- 
ety, and to great size, while reptiles 
diminish. 


Eocene. 


MESOZOIC 
TIME. 

Age of Reptiles. 


Cretaceous. 


Birds begin to become important, rep- 
tiles continue, and higher mammals be- 
gin. Land plants and insects of high 
types. 


Jurassic. 


Keptiles and amphibia continue to be 
predominant. 


Triassic. 


Amphibia and reptiles develop remark- 
ably. Mammals of low forms appear. 


PALEOZOIC 
TIME. 

The age of 
Invertebrates. 


Carboniferous. 


Land plants assume great importance. 


Devonian. 


Eishes begin to be abundant. 


Silurian. 


Invertebrates prevail, i 


Cambrian. 


No forms higher than invertebrates. 


In part 

AZOIC TIME. 

No fossils known. 


Archean.^ 


Mostly metamorphic rocks, perhaps in 
part the original crust of the earth. 



1 Invertebrates of course continue down to the very present ; but until 
the Devonian they were the most important group. The same is true of 
fishes, which begin to be abundant in the Devonian, but continue down to 
the present. 

2 From this group, some of the upper beds have been given a new 
name, the Algonkian, occurring just below the Cambrian. 



CHAPTER XV 

THE TVEARING AVTAY OF THE LAND 

Entrance of Water into the Earth. — When rain falls 
upon the surface, a portion of it runs directly away, and 
a part soaks into the ground. Each of these portions is 
engaged in the slow work of wearing away the rocks of 
the earth's crust. That part which sinks into the soil 
carries with it some of the oxygen and carbonic acid gas 
of the air, and perchance it also takes some substances 
from the decaying vegetation at the surface. When plants 
decay, carbonic acid gas is produced; and in the humus 
that is formed, there are substances, which when dissolved I 
by water, transform it either to a weak acid or an alkali. 
Wood ashes are often used in making soap because of 
their alkalies, and these are among the materials which 
the water obtains from the decaying vegetation. i 

Sinking through the soil, the water may encounter a I 
dense stratum, and while most will be turned aside from f 
its downward path, some sinks gradually into the rock, for in i 
every case the rocks of the surface are crossed by minute 
crevices. Some beds are much more porous than others, 
and some parts are more easily entered than others. 
Therefore there are paths along which more water runs 
than elsewhere. This is why wells dug in one place may 
not find a good supply of water, while those near by en- •: 
counter seams which furnish a permanent supply. Gen- [ 

240 



THE WEARING AWAY OF THE LAND 



241 



erally these water-producing seams are not the result of 
actual underground streams, but instead, are places in 
which water trickles through and between the rocks more 
readily than elsewhere. 

There is much difference in the permeability of the beds 
forming the crust. If water is poured upon sand, it quickly 
enters and disappears ; but if 
upon clay, the surface becomes 
wet, and very little water 
passes into the mass between 
the compact clay grains. Yet 
if Ave could examine these mi- 
nutely, we would find that 
some actually did enter. Even 
a dense rock like granite 
allows water to pass between 
and through the minerals. ^ 
Water exists all through the 
earth's crust as deep as man 
has explored; it is trickling 
through the strata, not only 
along the cracks and joints, 
but also into the very heart of the beds themselves. 

In its underground journey, water near the surface re- 
mains cool ; but it may pass near a lava which has been 
intruded into the crust, or it may settle down below 
the cold upper crust into the warmer portions, and in 
each case its temperature is increased. Laden as it is 




Fig. 118. 

Section of rock (gneiss) enlarged 
by microscope and showing 
cracks along which water per- 
colates. 



1 If a small piece of this, or nearly any other rock, be carefully dried 
for hours and then weighed, and afterwards soaked in water and weighed 
again, it will be found that it has become heavier as the result of the 
absorbed water. 



242 



FIRST BOOK OF PHYSICAL GEOGRAPHY 




Fig. 119. 

Diagram showing condition exist 

ing in some hot springs. 



with foreign substances (see above), the water, even when, 
cold, may do much work of solution and change ; but 

when warmed it has its power 
in this respect greatly increased, 
for warm water dissolves more 
easily than cold. 

Return of Underground Water 
to the Surface. Springs. — 
After a certain journey some 
of the water returns to the air. 
Perhaps it has gone far enough 
to have been heated, and then, 
when it flows out at the surface, 
its temperature may be as high 
as the boiling point. We then 
have a hot spring (Fig. 119), or if the water escapes by inter- 
mittent eruption, a geyser. Such springs have come to the 
surface from con- 
siderable depths, 
passing along a 
great break in 
the earth or a 
fault plane (Fig. 
114). 

More common- 
ly, however, the 
water seeps 
slowly to the 
surface, and it is 
this gradual seep- 
age that supplies the rivers. Indeed, if the rain that fell 
flowed away directly, the rivers would be violent floods at 




Conditions existing in hillside spring. P, porous rock ; 
I, impervious layers. Arrows indicate direction of 
flow of water. 



THE WEARING AW AT OF THE LAND 



243 



one time, and then as soon as the rain ceased, dry river 
channels. But a part of the rain water is stored in the 
earth, and gradually turned over to the streams after a 
short underground journey. Here and there, where the 
conditions favor, the water comes out as a spring (Fig. 120). 
There are many ways in which springs may be caused, but 
the most common is where water, passing through the soil, 
or a porous rock, encounters a less porous bed, along the 
surface of which it flows until it reaches the air. Such 
springs are very often located on hillsides, and sometimes 
the line of contact of sand and clay beds is marked by 
boggy places where the water is slowly escaping. ^^--^^^ 

Artesian Wells. — Men sometimes make springs artificially, and 
these are called artesian wells. Water, encountering a porous stratum 
which dips into the ground, follows it ; and if it is prevented from 
escaping by means of a more impervious layer, it may pass on down 
the incline. Such a stratum, becomes a water-bearing layer, from 




Fig. 121. 

Diagram showing conditions favoring artesian wells (A) in inclined layers ; 

porous (P) and impervious (I) . 

which, if a well is bored to it, the water rises, perhaps reaching the 
surface as an artesian well. The water is unable to escape because 
of the overlying beds which prevent it from rising, while the under- 
lying impervious layers prevent it from sinking. It is therefore 
under the pressure of the water above it in the inclined water-bearing 
stratum. That is to say, the pressure is great enough to force the 
water up through the well boring, nearly as high as the surface of 
the porous layer where the water has entered. Hence if a well is 
bored to this layer from a level below that where the water has 



244 



FIRST BOOK OF PHYSICAL GEOGRAPHY 




Conditions favoring artesian wells (A) in a 
syncline with porous and impervious (I) 
layers. 



entered the ground, the water will not only reach the surface, but 
wall rise into the air as a fountain. 

These are the conditions under which artesian w^ater is most com- 
monly found, and there are thousands of such wells in this country, 
and many more iu which the pressure is not quite strong enough to 

force the water out into the 
air, when it is necessary to 
raise it a short distance by 
pumping. A much rarer, 
but even more favorable con- 
dition, is that where the rock 
layers are bent into a syn- 
cline ; then there are two 
heads of supply, and no escape 
down grade ; for the fold 
forms a saucer-shaped depres- 
sion, w^hilein a singly inclined layer the water may pass downwards 
along the porous stratum which is dipping into the earth. In artesian 
wells the water may come to the surface scores of miles from the place 
where it entered. In eastern New Jersey and eastern Texas there are 
such wells, the source of whose water is scores of miles to the westward. 

Mineral Springs. — Many waters reached by wells or 
artesian borings, and many that come out as natural 
springs, have mineral properties. In other words, they 
have minerals in solution, and sometimes these are so 
abundant that the water has a very disagreeable taste. 
On the land, around many springs, deposits of minerals 
are being precipitated, because when coming out into the 
air, the water can no longer hold them in solution. One 
may find iron deposits of this origin around many iron 
springs ; and surrounding hot springs, where the water cools 
when it reaches the air, and therefore can no longer hold so 
much mineral in solution, there are sometimes extensive 
beds that have been precipitated, and to which additions 
are constantly being made. For instance, the hot springs 



THE WEARING AWAY OF THE LAND 



245 



of the Yellowstone Park are depositing extensive beds of 
carbonate of lime (Fig. 123) ; and from the water of the 
geysers, in the same region, silica is being precipitated 
(Chapter XX). 

This shows that in its journey, underground water is 
doing work of solution. Laden as it is with carbonic acid 
gas and other substances, it attacks the minerals and takes 
many substances away. These examples, which are im- 
pressive because they appeal to the eye, are in reality only 
extensive cases of exactly 

what all underground /^^ \^ 

water is doing. Not a 
drop of water passes into 
the ground, and escapes, 
without bringing to the 
surface a small load of 
dissolved mineral. 

It is this which makes 
water hard ; and a chemi- 
cal analysis of any spring 
or river water will reveal 
some iron, limestone, or 
gypsum, or all of these, 
and other substances as well. It is this which supplies the 
animals in the sea with the carbonate of lime which they 
need; and it is this which supplies the cement for the 
grains of the sedimentary rocks. One of the most impor- 
tant lessons taught by this solvent action, is that water 
in a river is always carrying something away. Every year, 
each large stream is bearing seaward thousands of tons 
of dissolved mineral; and this means that much solid 
jnatter is taken away from its drainage area. In the 




Fig. 123. 
Hot Springs, Yellowstone Park. 



246 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



course of the countless ages of geological time this small 
work amounts to a grand total of land destruction.^ 

Limestone Caves. — As there is a difference in the solu-. 
bility of minerals, so there is of rocks, some of which con- 
tain an abundance of soluble minerals. This is the case 



.,*^^ 



-m^ 



f-p 



Fig. 124. 

Howe's Cave, New York, showing stalactites on roof. 
S. R. Stoddard, Glens Falls, N. Y. 



Copyright, 1889, by 



with limestones in which caverns are being hollowed out 
by water action. Water, when sinking into the rocks, 
chooses some natural break or joint as the easiest way of 
entering the earth ; and slowly it enlarges this by solution. 
Then, perhaps coming to a more impervious layer, it passes 
along this, dissolving the limestone as it goes. At first 

1 The Mississippi River annually carries into the sea 150,000,000 toiig 
of dissolved minerals. 



THE WEARING AW AT OF THE LAND 



247 



the water slowly seeps through the rock along the numer- 
ous joints ; then as these become enlarged, and the pas- 
sage of the water is easier, the conditions may in time 
change until there is formed a veritable underground 
river, flowing in a great cavern which the water has dis- 
solved out of the rock^ (Fig. 
124). 

In a limestone country, like 
Kentucky near the Mammoth 
Cave, much of the water sinks 
into the earth, and small surface 
streams are rare, because the 
drainage is mostly underground. 
The surface water runs down 
into little saucer-shaped depres- 
sions, or hollows, where it cas- 
cades into the earth to take up 
its underground journey, emerg- 
ing, perhaps after a journey of 
miles, as a spring on the bank 
of a river. Gradually the sur- 
face of the land is being worn 
down, and in time these caverns are partly opened to the air, 
and may be entered, as are the Mammoth, Luray, and many 
others. In time this would go so far that the underground 
river becomes a surface stream, because the roof of the cave 
has disappeared; but before this open-stream condition, there 
might be a natural bridge formed, where a part of the cave 
wall, firmer than the rest, has been left standing as a bridge 
across the valley, the last remnant of the old cavern. 




Fig. 125. 
The Natural Bridge, Virginia. 



1 This is the case in Mammoth Cave, where not only is there a river, 
but one in which fishes live. 



248 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



The water also slowly percolates into the limestone rock through 
many smaller crevices, and enters the cave through the roof, drop- 
ping from this to the floor. On this journey through the rock it has 
taken some of the carbonate of lime into solution ; but upon entering 
the cave, some of the carbonic acid gas, which permitted the water to 
dissolve the limestone, escapes into the air, and the water is then 
unable to hold all of the lime in solution, 
and is therefore forced to deposit some of its 
mineral load, either upon the cave roof or upon 
the floor below. Little by little these deposits 
grow, forming pendent icicle-like columns, or 
stalactites, from the roof (Fig. 124), and smaller 
stalagmites from the floor. In time these may 
unite, forming columns (Fig. 126) ; and the 
weird and even beautiful way in which these 
deposits increase, often ornaments the caverns 
so that they become places of distinct interest 
and beauty, as in the case of the remarkable 
Luray Cave. 

Breaking up of the Rocks. Methods 

Employed. — By the action of solution 
described above, rocks are being slowly 
disintegrated; but there are other 
actions co-operating to cause the rocks 
to crumble. If this were not so, much of the earth would 
be in the condition of bare rock, instead of a soil-covered 
land; and the rivers would not be furnished with sediment 
to transport to the sea. The strata that form the land 
are hard, and they must be softened and divided into 
bits before they can be worn away and carried from land 
to sea. This process of decaying, softenings and crum- 
bling the rocks is commonly called weathering^ because 
it is due to the action of the weather. 

One of the most potent agencies of rock disintegration 
is water. This is at work changing and dissolving as it 




Fig. 126. 

Column in a cavern, 
caused by union of 
stalactite and sta- 
lagmite. 



THE WEABING AWAY OF THE LAND 



249 



passes through the crevices of the rock. Every bit that 
is taken away in solution weakens the mass, for then the 
minerals are less firmly supported, and in time they may 
fall apart. As it passes along, the water finds many min- 
erals ready for change, as iron is when exposed to damp- 
ness. Sometimes they need oxygen, sometimes carbonic 
acid gas, sometimes water, and very often two or all of 
these, and perhaps other substances which the water is 
bearing. In this way hard minerals are softened and 




Fig. 127. 
Effect of frost action on mountain top in Colorado. 

rocks crumbled. The changes that take place in the 
minerals are very complex, and one of the results of 
these is that substances are produced which the water can 
then take away in solution. 

In cold climates, and particularly on high mountain 
tops, and in the high temperate and Arctic latitudes, water 
in the rock crevices sometimes freezes at night and thaws 
in the daytime. When ice is formed, an expansion takes 
place, and being confined in small crevices, the ice presses 



250 FinsT BOOK OF PHtStCAL GEOGBAPBY 

with great force against the enclosing walls, with the 
result that tiny fragments and even great pieces are some- 
times pried off. Not only does this prying apart of the 
rocks break them up directly, but it opens cavities into 
which water more readily enters, and this increases its 
power of dissolving and changing the minerals. Frost 
action is everywhere at work where rocks and soil are 
exposed to the air in cold climates ; but below the depth 
of a few feet it loses its importance, because the changes 
of temperature do not extend far into the earth. Therefore 
the soil, even though no deeper than two or three feet, 
serves as a blanket to protect the rock below. 

In hot countries the change of temperature from day to 
night causes expansion during the day and contraction at 
night; and this, which is necessarily different in different 
parts of exposed rocks, causes bits to snap from them. On a 
much larger scale this action may be seen when a fire is built 
against a stone, or when a brick or stone building burns.^ 

Plants are also helping to destroy the strata. In their 
sap they take mineral substances from the soil, and upon 
dying they furnish carbonic acid gas, humic acid, and 
other substances to the water, which sinks into the earth 
through the mat of vegetation. They are also aiding 
mechanically ; for as they grow, their roots and tiny root- 
lets ramify through the soil, and even enter the rock itself, 
which they pry apart as they grow. Every tree, every 
blade of grass, and every lichen that clings to the surface 
of a boulder or ledge, is engaged in pulverizing the rock 
or the soil. 

1 Or it may be illustrated by placing a lighted candle beneath a piece 
of window or bottle glass, or even a chip of rock, which will soon crack 
because of the unequal expansion in different parts. 



Tat] WEABiNG AWAY OF THE LAND 251 

Difference in Result. — There is a great difference in the 
power of this weathering. Some strata are so porous that 
water easily enters and causes the changes mentioned ; but 
others are dense and difficult to penetrate. Many are made 
of insoluble or nearly insoluble minerals, and others are 
easily destroyed by solution. Some rocks, neither porous nor 
soluble, are made of minerals which decay rapidly. Where 
a rock is porous, and has some minerals that can be easily 
dissolved, and others that are readily changed or decayed, 
the rate of weathering becomes much more rapid than 
where only one or none of these conditions are favorable. 
Hence since rocks differ in composition, one sees, side 
by side, layers that are worn away rapidly and beds that 
resist the weather. 

This is one of the most important principles of the 
physical geography of the land ; and it accounts for many 
of the hills, mountains, and valleys. For instance, Mt. 
Washington, the peaks of the Adirondacks, Pike's Peak, 
and many other elevations, are made of granitic rocks 
which are more durable than others surrounding them; 
and while neighboring strata have been crumbled and 
carried away, they have resisted and stood up, ever 
becoming higher above the neighboring country, not so 
much because they were lifted there at first, as because 
they have remained more nearly at the elevation to which 
they were raised. In the same way the ridges of the 
Appalachians are often made of durable sandstone and 
conglomerate strata, while the valleys are frequently 
located in beds of more easily removed shale and lime- 
stone ; and all over the earth's surface similar illustra- 
tions, great and small, may be found. 

There are also differences according to the conditions 



252 



FinsT BOOK OF PBTStCAL GFOGRAPBY 



which surround the rocks. Climate is one of the most 
important of these (Figs. 128 and 129). A moist region 
furnishes more water to perform the work than does an 
arid climate, and hence the rocks melt down less rapidly 
in the latter than in the former regions. Moreover, vege- 
tation is more luxuriant in a moist climate than in an 
arid one, and this furnishes to the water the tools with 




Fig. 128. 

Effect of weathering upon a hill in the arid regions, where the horizontal 
strata have been carved by rain action because of the absence of protection 
by plants. 

which it works in rock decay. In a very cold climate 
frost is active and weathering rapid. In a warm country 
this condition is absent ; and although there is more water 
action, its effect is not equal to that of frost. 

In such cold lands as the Arctic regions, or the high 
mountain peaks, altitude is another modifying condition. 
Upon a mountain top, where the winds are violent, and 
where the slope is so great that the rain and melting 
snows run down the mountain sides with great velocity, 



THE WEABING AWAY OF THE LAND 



25S 



the tiny bits of rock, broken off by weathering, are quickly 
removed, and so the rock remains bare and open to the full 
effect of the weather (Fig. 127). When this is combined 
with a cold climate, the rate of weathering is greatly 
increased. 

The slope of the land is another feature. This has just 
been mentioned in speaking of mountains ; but in this place. 




Fig. 129. 

A mountain (a butte) in western Texas, showing cliff exposed to weather 
in an arid climate where vegetation is scanty. 



we may also contrast the precipice with the gently sloping 
hillside. In the former case, every piece that is loosened 
by frost, or any other cause, falls from the cliff, leaving the 
face bare to the attacks of the weather ; but on the more 
gently sloping hillsides many of these fragments remain, 
and soon, by accumulating, form a soil which to some 
extent protects the rocks from the action of the weather 
(contrast Figs. 136 and 139 with 142 and 143). 



254 FIRST BOOK OF PBTSICAL GEOGBAPHT 

Effects of Weathering. — We may watch a cliff or a rock 
for years without seeing any notable change; yet if one 
looks at its surface it is seen to be rough and crumbly, while 
within, the rock is fresh and hard.^ Many stone buildings 
have crumbled at the surface in the 200 or 300 years 
since they were built; and the obelisk, brought to the 
damp, changeable New York climate, has been disinte- 
grating so rapidly that it has been found necessary to 
protect it from the weather. Taking into consideration 
all the time during which these agents have been at work, 
not merely a few years, but tens of thousands or even 
hundreds of thousands of years, the slow work of weather- 
ing becomes very important in its effect. It is revolu- 
tionizing the outline of the land ; and again and again 
mountains have been raised and worn down, valleys have 
been dug where hills once existed, and the face of the land 
has been changed, and is even now very slowly varying in 
its features. In this work of change the wind and rivers 
have aided, but weathering has been of prime importance. 
If weakness exists in rocks, this delicate tool will find it : 
and while the weak part is rapidly destroyed, the hard 
portions stand out in relief. Therefore one of the most 
important effects of weathering is the sculpturing of the 
land. 

A second important effect is the supply of materials to 
the rivers and the ocean. In a gorge the rock fragments 

1 This may be best seen in a quarry by contrasting the fresh rock, that 
is being quarried, with the decayed surface. Probably the outside is dis- 
colored, perhaps by an iron stain, because by the decay of the minerals 
which contain iron, this change becomes visible, while others, perhaps 
even more important, are not noticed. By direct observation one may 
see that these rocks are crumbling ; and geologists find abundant evidence 
that all have been and still are being disintegrated. 



THE WEARING AWAY OF THE LAND 



^^5 



are constantly dropping to the bottom of the cliffs (Fig. 
130), where they either enter the stream, and are taken 
along with the current, or if more material is supplied 
than can be carried, or the fragments passing down the 
cliff fail to reach the stream, accumulate at the base of 
the precipice, forming a talus deposit (Fig. 134). A sea 




Fig. 130. 

Crumbling of rocks on a mountain side, showing the sliding down of the frag- 
ments which fall from the cliffs. 

cliff furnishes debris to the sea in the same way. We 
can see the importance of this, and hence it appeals to us ; 
but more important still is the slower and less perceptible 
down-sliding of the soil fragments on the more gently 
sloping land. It is not more important because more 
rapid, but because the area of such slopes greatly exceeds 
that of steep cliffs. Every rain is washing some material 
down the hillsides which border the river valleys. This 



256 



FIRST BOOK OF PBTSICAL GEOGRAPHT 



is why the streams become muddy with sediment after 
heavy rains and during the melting of the snow. 

This load of sediment is important for several reasons. 
First, it keeps the weathered material from accumulating 
on the rock and protecting it by a deep blanket. Hence 
the removal aids weathering ; for if some were not removed, 
the rock would soon be covered to such a depth that frost 
and plants would produce no effect, and even percolating 

water would not be of 
great importance. It is 
also important because 
it gives sediment to the 
sea, where it is built into 
the beds of rock, which 
later, raised to the sur- 
face, form such notable 
elements of the land. 
Then too, the prepara- 
tion of material by 
weathering furnishes 
streams with tools with 
which to work in cutting 
their valleys. Water by itself has little power to cut the 
rocks; but armed with pebbles, sand, and clay, the stream 
rasps at its bed and slowly deepens its channel (Ch. XVI). 
To man the most important effect of weathering is the 
formation of soil. The crumbling of the rocks furnishes an 
accumulation of soil fragments into which plants can 
easily thrust their roots ; and the decay of the minerals 
furnishes substances which are needed in plant growth, 
and which, being soluble in water, are taken from the soil 
in the sap. This soil which is formed by the decay of 




Fig. 131. 

Decaying granite, Maryland. Fragments 
of rock and clay, formed by decay, sur- 
round blocks not yet reached by weather- 
ing, and hence fresh enough to be quar- 
ried. 



THE WEARING AW AT OF THE LAND 



257 



rocks, and which is the most common one in the world,^ is 
called residual soil. It is one from which many of the 
soluble substances have been removed, and is hence com- 
posed chiefly of the insoluble residue^ particularly kaolin 
clay and silica. In such a soil the surface portions are 
very fine-grained clay, grading downward to fresh rock. 




rTn5^ FRESH 
J -r-i rJ r^ M ROCK 



Fig. 132. 
Diagram to show the conditions in a region covered by residual soil. 



passing first through a zone of partially decayed rock 
fragments in a clay matrix, and then to soft and partially 
deca3^ed beds, not yet pulverized to fragments. Such soils 
may reach 100 or 200 feet in depth, though they are com- 
monly much less. 

Erosion of the Land. — The land is being destroyed by 
other agents, which grind down the surface and remove 
the fragments by erosion. Here and there glaciers pass 
over the land (Chapter XVII), and as they slowly move 
along, dragging soil and rock fragments with them, they 
scour the beds over which they pass, grinding them down 
and carrying the pieces thus wrested to their ends, where 
they may be left on the land or carried away in the streams 

1 In northern United States and Europe the soil has been brought by 
glaciers ; and along rivers there are soils which have been accumulated 
by the water. Some are also formed by wind action, and some were 
once deposited in the sea, and have since been raised above it. 



258 



FIRST BOOK OF PHYSICAL GFOGBAPHT 



produced by the melting of the ice, or often, where the 
end is in the sea, be borne away by icebergs. Another 
agent of erosion is the wind^ particularly in arid countries, 
where the soil is not held in place by abundant plant life 
(Fig. 133), and where, because of the dryness of the climate, 




Fig. 133. 
Rain-sculptured Bad Lands of South Dakota. 

the soil particles do not cling together. Along the coast 
line the waves and tides are ever at work destroying the land 
and removing the fragments (Chaps. XIII, XVIII). Be- 
cause of the great activity of the destructive agents in the 
sea, this is one of the most rapidly changing parts of the land. 
On the land, water is also active in erosion. Every 
heavy -rain removes some material from the surface ^^nd 



THE WEARING AW AT OF THE LARD 259 

carries it to the rivers. The raindrops, first striking a 
blow upon the soil, gather into tiny rills, and these form 
streams; and from the first impact of the raindrop, the 
water may be engaged either in the removal of loose 
particles, or the grinding of hard rock. The erosion by 
rain itself becomes more important when vegetation is not 
present to check the force of its fall and to hold it, pre- 
venting its rapid escape. Therefore, in roads, ploughed 
fields, and in the great desert and semi-desert countries, 
rain erosion becomes very noticeable (Figs. 128 and 133). 
Gathering into rivers, the rain-water becomes more con- 
centrated, has its power increased, and the beds of the 
streams may, therefore, be places of very rapid erosion 
(Chapter XVI). Valleys are cut, and gorges and even 
deep canons are carved in the rocks. This is one of the 
most potent agents in the sculpturing of the land. This 
work of rivers goes hand in hand with that of weather- 
ing; for the latter, by causing the rocks to crumble, fur- 
nishes rivers with tools and materials to carry. 

Much of the rain that falls on the land sinks into the ground, and 
while there it not only dissolves and changes, but also accomplishes 
much mechanically, as an agent of erosion. The underground water 
lubricates the soil particles so that they slide down the hillsides ; and 
this is one of the most important of the effects of percolating water. 
Gradually the soil migrates down hill even if the slope is not very great. 

Also, water percolating along the contact between a porous and 
a clayey layer, makes the surface of the latter slippery, so that if the 
slope is steep, the porous bed may commence to slide, perhaps form- 
ing a tiny landslip, or landslide, though sometimes, on steep mountain 
sides, a great and destructive avalanche. These slips of the land, which 
sometimes carry thousands of tons of soil and rock, are often caused 
when the frost is coming from the ground and the earth is then made 
damp. Or perhaps a heavy fall of snow will increase the weight of 
an unstable part of the hill or mountain side, causing an avalanche, 



260 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Destruction of the Land. — The combined work of 
weathering and erosion is called denudation. By these 
agents, slowly operating but ever at work, throughout 
the long periods of time during which the land has been 
exposed to the air, the most profound effects have been 
produced. Not merely has the land been sculptured into 
its present outline of hill and valley, but great mountains 
have been reduced to lowlands, and thousands of feet of 
rock have been removed from the surface. Denudation 
is engaged in the great task of destroying the land and 
transporting to the sea the materials thus derived ; and if 
it had been permitted to work uninterruptedly through- 
out all past time, the land long before now would have 
been reduced to a nearly level surface. 

But it is not permitted to work without interruption. 
The land is rising here and falling there ; and as a result 
of the contraction of the heated globe, the land sur- 
face is steadily rising, though now and then locally sink- 
ing, while the bed of the ocean is gradually becoming 
deeper. Therefore, while the land is being attacked 
by denudation, it is also rising ; and we may be certain 
that this uplift has been more rapid than the down- 
cutting caused by denudation, otherwise the surface 
would be less rugged and the level lower. The two 
are in conflict, and so far denudation is the weaker of the 
combatants ; but as a result of the conflict, the face of the 
land has been battered and carved into the irregularities 
of seashore, plain, plateau, hill, valley, and mountain. It 
will be interesting to look a little more closely at some of 
the methods employed in this battle, and at some of the 
results which have been produced, 



CHAPTER XVI 

RIVER VALLEYS, INCLUDING WATERFALLS AND 
LAKES 

Characteristics of River Valleys. — Rivers occupy valleys, 
and among these there is an extremely great variety of 
form. Some are narrow, some broad, some deep, and some 
shallow. In every single case there is a certain relation 
between the conditions surrounding the river, and the 
form of its valley. In such an elementary book as this 
we can do no more than understand some of the simplest 
principles, though geologists studying a river valley can 
generally find out why it has its particular form. Some 
valleys are situated in easily destroyed rocks, some in 
durable layers, and some cross first one and then another. 
Many are situated on plains, others in mountains and 
plateaus, and some exist in moist, others in arid, climates. 
In many cases rivers have occupied their valleys for a very 
long time ; but a few have existed for only a short period. 
With all of these variations, there is a resulting variety in 
river-valley form ; but there is one characteristic of all 
rivers, — that they flow in some kind of a valley. 

Another universal fact is that at some time all river val- 
leys contain water. In most of them some water is always 
present, sometimes a very small amount, sometimes veri- 
table floods ; but there are some valleys which contain 
water only for a very short time during the year, and 

261 



262 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



others which may remain dry for many years. That is to 
say, while water is in every case sometimes present, in all 
cases the amount varies from time to time. The velocity 
of the stream differs with the slope of its bed, and some 
have steep, others gentle slopes; but with any given 
grade^ the velocity of the water also varies with the 

amount of water. To 
prove this let any one 
examine a stream that 
flows quietly along in 
ordinary times, and con- 
trast this with the tor- 
rent that rushes over 
the same slope after a 
heavy rain, or when the 
snow is rapidly melting. 
In all rivers this water 
supply comes partly 
from the direct fall of 
rain, and partly from un- 
derground water which 
once fell as rain, and 
, ^ . , . ,/°'. "^^^ ^ . ,., , . ^ then entered the earth. 

A typical river valley m mountains (the nigh 

Andes of Peru), showing gorge cut hy All riverS have tribu- 

rapidly descending stream, and also talus f^^.^ ^^^ ^^lese vary 

supplying debris from the valley walls. ^ ' "^ 

in number and kind. 
Perhaps some tributaries are great rivers, like the Ohio 
where it enters the Mississippi, but most are only tiny 
rills which exist during rains. Every river occupies 
a certain basin or drainage area^ and the combination of 
all the streams in this area forms the river system. Here 
too, there is great variety in form, size, and condition. 




BIVER VALLEYS 263 

Two neighboring river systems are separated by a line, or 
more commonly by an area, known as a divide or water 
parting. This may be a sharp mountain ridge, or much 
more commonly, a gently rounded hilltop, or even a 
swampy plain. 

Most of these features are ever changing ; for the val- 
leys, valley walls, tributaries, divides, and areas of the 
river systems are caused to be what they are by the com- 
bined action of running water and weathering. That this 
is so, is proved by the fact that all rivers are at some time, 
and many are at all times, carrying loads of minerals in 
solution and rock fragments in suspension. A river is 
nothing more than a drainage line on the land, by which 
the surface water is passing from high to low ground, 
generally toward the sea.^ In its course, because of the 
co-operation of weathering, and by its own action, the river 
is obtaining mineral matter to remove from the land 
(Fig. 134). Hence it also becomes a carrier of fragments 
obtaiued from the waste of the land. Incidentally, be- 
cause of these two facts, the river is grinding a valley. 
That is to say, the water flows down hill, and is furnished 
with rock fragments ; and with these in its grasp it scours 
its bed, ever deepening it when this is possible. There- 
fore, according as the slope, volume of water, amount of 
sediment, kind of rock, and length of time in which the 
work has proceeded, vary from place to place, the form 
6i the river valley also varies. This point of river varia- 
tion from time to time and from place to place will be 
considered in some detail. 

1 In enclosed basins, like the Great Basin of the west, or the Dead Sea, 
the water flows into an interior area and hence not necessarily tow9,r4 
t|ie oce£{.n. 



264 



FIRST BOOK OF PHYSICAL GEOGBAPRT 



The River Work. — When water gathers into a stream, 
and courses along down a slope, it cuts into its channel 
by two kinds of action. It dissolves such substances as 
it can, and it scours its bed by dragging rock fragments 
along. The rate at Avhich it will do this varies with the 
velocity of the water, the amount of sediment, and the 
kind of rock over which it flows. Limestone is dissolved 




Fig. 135. 
A stream bed in the Adirondacks, showing boulders that are moved by the 

floods. 



with greater rapidity than granite, and a soft clay bed 
will be worn away more rapidly than a hard rock. 

To do this work most rapidly the stream should have 
sediment with which to wear away its channel ; but some 
streams have too much sediment, in fact more than they 
can carry along; and then some of the load must be de- 
posited in the bed instead of being able to cut into the 
rock. The Platte in Nebraska for instance, although 



liIV:ER VALLEYS 



265 



flowing down a moderately steep grade, is not cutting a 
valley, but is building up its bed. On the other hand, 
Niagara River, 
above the Falls, has 
so little sediment 
that it is not able 
to scour its channel. 
When this river 
flows out from Lake 
Erie, it starts as 
clear lake water, 
and as a result of 
this, the stream be- 
low Buffalo flows 
almost at the sur- 
face of the plain. 

A stream that 
flows over a gentle 
slope is less able to 
cut into its channel 
than one that passes 
down a steep grade, 
because it hurls 
rock - bits against 
its bed with less 
force ; and since the 
velocity becomes 
greater when the 
volume of water in- 
creases, it can cut 

into its bed more rapidly when in flood than at other times. 
To appreciate this, one has but to watch the rushing torrent 




Fig. 136. 
View in Enfield gorge near Ithaca, N.Y., show- 
ing young stream deflected by joint planes. 
Part of a circular pot hole in the foreground. 



266 FIBST BOOK OF PHYSICAL GEOGBAPHT 

which courses down a stream valley in the spring, and see 
it carrying along pebbles, and even bonlders, which under 
ordinary conditions it would not be able to move, and 
which, when the flood subsides, are left standing in the 
channel (Fig. 135). The greater part of the cutting done 
by most streams during the year is accomplished during 
the few days when the water flows as a torrent. For the 
remainder of the year, though a small amount of work 
is done, the stream loiters and rests from its labors. In 
a year there has been no perceptible deepening of the 
stream channel ; but in a few centuries rapidly working 
streams will make changes ; and in the great ages of geo- 
logical time, vast results in valley formation have been 
accomplished. 

If we should go to any stream valley, where the water 
is flowing over the bed rock, we would find that in certain 
places, perhaps where the rock is soft or much jointed, 
the channel had been turned aside for some distance (Fig. 
136), or that the water had cut into those places more 
deeply than elsewhere. Perhaps in places where the flow 
was increased for any reason, circular pot holes had been 
dug (Fig. 136). These are carved at the base of water- 
falls or in an eddy of the stream, where the swirling 
waters have whirled the pebbles about (Fig. 139). In 
such river beds there is proof that the stream is really cut- 
ting into its channel ; for one can see that a hole has been 
dug, or that blocks have been cut out; and one may also 
see that softer rocks are more rapidly worn than harder. 

Most valleys have greater breadth than depth, and the 
width of the valley is very much more (perhaps miles) 
than the width of the stream channel. So far as we have 
gone in this study, we have seen only that the stream cuts 



mVEU VALLEYS 



267 



ORIGINAL 



its bed; and if this were the whole truth, a river valley 
should be narrow, its width being about that of the stream 
itself (Fig. 137). Evidently therefore, there are other 
facts to be un- 
derstood. One 
of these is that 
the river does 
not flow over a 
straight course, 
but meanders 
about (Fig.138). 
This is true of 
all streams, and 
it is also true 
that in meander- 




FiG. 137. 

Diagram to show relatively small amount of rock 
actually cut out by the stream, compared with the 
width of the valley as broadened by weathering. 



ing they change their position from time to time. Com- 
monl}^ the place of greatest velocity of a river is in the 
middle ; but when it begins to swing, the place of great- 
est velocity shifts first to one side of the valley, then to 
the other. Hence the current strikes against the hank 
(Fig. 138), and in addition to the vertical cutting in the 
stream bed, there is a certain lateral cutting against the 
valley walls. Since the swing of the river changes from 
time to time, different parts of the side are successively 
attacked, and so by this action the valley is slightly 
broadened (Fig. 138). 

The really great tuidening of valleys is that done by 
the very slow action of weathering (Fig. 134). This is 
always at work, and little by little the rock crumbles, 
passes into the stream (Fig. 130), and is whirled off 
toward the sea. Every block that falls from the cliff, 
every mud-laden rill that courses down the hillside, and 



268 



FIBST BOOK OF PHYSICAL GEOGBAPHT 



every bit of dissolved substance brought to the surface 
by underground water, represents a small contribution 
to the grand sum total of valley broadening. To appre- 
ciate this we must remember that the valley has been de- 
veloping, not for a century only, but for many tens or 
even hundreds of thousands of years. 




Fig. 138. \ 

Part of map, showing meandering river cutting against bluff on one side and j 
depositing on opposite. Arrows indicate the strongest current. 

History of River Valleys. — If a river were supposed to 
begin upon a new land, the water would take the lowest 
course, and along this ^ would commence to dig a valley. 
Weathering would of course immediately begin to co-oper- 
ate ; but for awhile the work of the river in cutting its 
channel would be more rapid than the action of weathering 
in widening the valley, because the body of water moving 

1 Called the consequent course, because it is chosen in consequence of 
the topography. 



RIVER VALLEYS 



269 



along the narrow channel is a more powerful agent than 
the very slow action of weathering and rain wash. Con- 
sequently the valley would be narrow and its sides steep, 
because the river 
carves the rock with 
such relative rapid- 
ity (Fig. 137). Such 
a stream valley 
would be a young 
valley (Figs. 134, 
136, 139, and 145); 
and wherever we 
find a gorge or canon, 
we may be certain 
that it has not existed 
long enough for 
weathering to widen 
it. 

A young river val- 
ley may have a very 
irregular course, for 
it has taken the low- 
est line, following it 
wherever that may 
lead ; and many young 
streams pursue a very 
roundabout path to 
their mouths. It 
may also have lakes 

in the course, for there may have been depressions in its 
bed which had to be filled up before the river could pro- 
ceed. As time goes on, these will be destroyed; for each 




Fig. 139. 

A young valley being cut in the shale rock of 
central New York. 



270 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



stream that enters a lake, brings sediment which is slowly 
filling it; and at the same time the outlet stream is cut- 
ting its valley down, and therefore lowering the lake 
level. In time the combination of these causes succeeds 
in removing the lake, and hence lakes are not liable to be 
found in valleys that have passed the youthful stage. 
Waterfalls also may exist in the course of a young stream, 
where its path has led it down some steep slope ;i and 
falls may be developed as the river cuts its bed, passing 




Fig. 140. 
Diagram to illustrate base level (B E) and profile of equilibrium (P E) . 



over hard or soft rock. Therefore a gorge valley, with 
lakes and waterfalls, is characteristic of young streams. 

After awhile, as the stream valley becomes older, and 
reaches the stage of earlT/ maturity^ there are differences 
in the conditions, and therefore in the form of the valley. 
There is a level below which the stream cannot cut. At 
its mouth, where it enters the sea, this is the sea level, 
and it is commonly called the base level of erosion (Fig. 
140). The sea level is the permanent base level of streams, 
but temporarily there may be a base level above this. For 
instance, so long as a lake exists in the course of a stream, 
its channel cannot be cut below the temporary base level 
of the lake surface. While a stream may cut its channel 

1 As in the case of Niagara. See latter part of chapter. 



mVMB VALLEYS 



271 



down to sea level near the sea, it can never do this far 
inland, for the river must maintain a slope down which 
the water can run, and at the same time transport its 




Fig. 141. 

Diagram to illustrate the development of a stream valley from original sur- 
face (ee'), through youth to old age (gg'). 

sediment load. This line, sloping down toward the sea, 
may be called the profile of equilibrium (Fig. 140). ^ 




Fig. 142. 
A broad, mature valley, Ithaca, N.Y. 

When a stream in its down-cutting has reached nearly 
to this lowest possible slope, the profile of equilibrium, 

1 Called this because it is the profile in v^hich an equilibrium is main- 
tained between water and sediment supply, and river slope. 



272 



FIB ST BOOK OF PHYSIC AL GFOGBAPBT 



its power to cut further is very greatly diminished, and 
finally, when the profile is actually reached, it ceases to 
be able to cut into its channel. But weathering still 
continues, and so while the valley is no longer being 
deepened, it begins to grow broader, and the gorge or 
canon gives place to a valley with rounded sides (Fig. 
141). This is characteristic of the stage of maturity 
(Fig. 142). 





Fig. Ii3. I 

A view on the Delaware, showing the Water Gap in the background, where | 

the rocks are hard conglomerate, and the broad, gently sloping valley in j 
the foreground, where the rocks are softer limestones and slates. 



The length of time which it will take to reach matu- 
rity varies greatly, as it does in animals and plants : in 
some climates weathering is slow, in others rapid ; in some 
places the stream begins high above sea level, and there- 
fore has more slope, and more work to do, than others 
which begin on low plains ; and some work in hard, others 
in soft rock. Indeed, two neighboring parts of the same 
stream may show different stages of development, one 
being in soft rock, the other in hard, and hence one 



BIVEB VALLEYS 273 

having a more rounded outline than the other (Fig. 143). 
Also the headwaters of a stream will be less mature than 
the lower portions, for these are higher, carry less water, 
and have more work to perform. Moreover, they cannot 
be developed /as^er than the lower portions, for they must 
wait for these in order that they may have the necessary 
slope down which to carry their sediment supply. 

There are other ways in which a mature valley differs from a 
young one. Waterfalls cannot exist, because the slope is the easiest 
one possible, and all falls have therefore been destroyed, ^or can 
lakes exist, for they have all been filled. The river course may have 
become quite different from the original, for in time rivers adjust 
themselves, and often gradually alter their direction. Also at first 
the divides may have been very indefinite and the tributaries few ; ^ 
but the tributaries increase in number, and during maturity the 
divides are so definite, that all water falling on the land finds a slope 
down which to flow, and a valley in which to join with other water 
to ultimately reach the main stream. 

After the profile of equilibrium is reached, the operation 
of weathering slowly continues to broaden the valleys and 
lower the hilltops, until if everything is favorable, the 
country will be reduced to the condition of a plain 
(Fig. 141). There can be no doubt that there has been 
time enough for this; but there are no such old valleys; 
and here is an illustration of the combat between the 
elevating and destroying forces. The land is rising 
faster than denudation can remove it, and hence there are 
no really old lands. This which has been stated is the 

1 This is well illustrated in both the Red River valley of the North, and 
in Florida, which are young plains. Here the divides are great, nearly 
undrained swamps, and the water that falls scarcely knows which way to 
flow. In time, as definite courses are chosen, other streams develop, and 
gradually the divides become more sharply defined. 



2T4 



FIBST BOOK OF PHYSICAL GEOGEAPHY 



normal or ideal cycle of change. In reality rivers are sub- 
ject to many interruptions, which may be called accidents^ 
and these will now be considered. 



Fig. 144. 
Diagram to show young, inner, and older outer gorge of Colorado River. 

Accidents interfering with Valley Development. — The 

country in which a valley is being formed may be raised, 

and this gives 
to the stream 
new power to 
cut, for it in- 
creases the 
slope. By this 
the stream is 
rejuvenated o r 
revived^ and the 
gorge condition 
may be pro- 
longed, or a 
gorge may be 
cut in the centre 
of the mature 
valley, as in the 
case of the Col- 
orado River of 
the west (Figs. 
Here the river had cut down to its profile 




Fig. 145. 

A view in the Colorado canon, showing inner gorge 
and outer broad valley. 



144 and 145). 



RIVER VALLEYS 



275 



of equilibrium, and the valley sides had wasted back, 
forming a fairly 



broad valley. Then 
there came an up- 
lift of the land, 
since which the 
Colorado has cut 
a narrow canon, 
which is a deep, 
narrow trench in 
the middle of the 
older, broad val- 
ley. If such an 
elevation should 
occur along the 
coast line, separate 
streams might be 
caused to unite 
into one. For in- 
stance, if the re- 
gion about Chesa- 
peake Bay were 
to be raised to an 
elevation of 200 
or 300 feet, many 
streams now enter- 
ing this by sepa- 
rate mouths would 
be united, flowing 
to the sea through 
a single trunk stream (Fig. 146). 
occurred in the past. 




Fig. 146. 

Map of Chesapeake Bay, to show (by heavy line) the 
way in which the various rivers would unite into 
a single trunk stream if the land were elevated. 



Such elevations have 



276 FIRST BOOK OF PHYSICAL GEOGBAPHT 

If the reverse movement of depression takes place, the 
stream loses most of its power, because its slope is 
decreased; and then it may build a broad floodplain 
(Fig. 138), because it is no longer able to carry all its 
sediment, but must deposit some in its bed or to one side 
of the channel. If in this case the depression is near the 
sea, the stream valley may be submerged or drowned, and 
in fact this is the cause of many of the bays, harbors, and 
estuaries along the coast (Plate 19). In these the sea has 
extended up the valleys, and this has separated or dissected 
rivers which once entered the sea by a single mouth, but 
which now, as in the case of the Chesapeake, enter the 
sea by separate mouths (Fig. 146). During recent geo- 
logical times northern Europe and America have been 
depressed, so that the sea enters many of the valleys, 
transforming the coast to one of extreme irregularity 
(Plate 19). 

Another way in which the normal development of the 
stream valley may be interfered with by movements of 
the land, is when the surface changes in level along a 
relatively narrow line. For instance, a mountain chain 
may be rising across a river valley. In this case perhaps 
the stream ^\\\ maintain its course, cutting into the rock 
as rapidly as the mountain rises. Such a stream, called 
an antecedent river, because it existed before the mountain 
grew, would then cross the mountains directly, forming a 
deep mountain defile or gorge. Though many rivers cross 
mountains, as in the case of the Delaware at the Water 
Gap (Fig. 143), the Susquehanna, and others in the Ap- 
palachians, it is very difQcult to prove that the cause for 
this has been that just mentioned. In the main, such 
mountain valleys are the result of later changes, and this 



BIVER VALLEYS 277 

applies to practically all in the Appalachians ; but some 
believe that the Green River, where it crosses the Uinta 
Mountains in Colorado, is an antecedent river. 

Much more commonly the growing mountain dams the 
stream, forming a lake which may later be filled up by the 
river sediment, after which a gorge may be cut at the point 
of overflow ; which may be the same as the original course, 
or may be in a different direction. No doubt during 
mountain formation the land often rises so rapidly that 
the stream is turned to one side or diverted, or perhaps 
even forced to flow in the opposite direction, having its 
course reversed; and the mountains may rise rapidly 
enough to divide a stream into two parts. In any event, 
the growth of a mountain seriously interferes with the 
development of the valley. 

Climates have changed in the past. In the Far West, 
places now arid once had a moist climate. Some streams 
in this region that were formerly larger, are now shrunken; 
and in some places, where the weathering of the damp 
climate was rapid, there is now relativel}^ little weather- 
ing, and the broadening of valleys is therefore a slow 
process, so that the typical valley of the arid land is 
an angular canon (Fig. 145). In this western region the 
change in climate has caused some valleys to be entirely 
abandoned, and some streams to be dissected. For instance, 
the rivers now flowing into the Great Salt Lake valley once 
united to form a larger lake, which overflowed into the 
Columbia, and thence to the sea; but now they all run 
down into the Great Basin, where they disappear by 
evaporation. Hence many streams, at present having an 
entirely separate existence, and distinct mouths, were 
once united into a single stream. 



278 FIRST BOOK OF PHYSICAL GEOGRAPHY 

One of the most important accidents to rivers is that caused by 
the great glaciers which once overspread northwestern Europe and 
northeastern America. By this nearly all of the rivers have had 
their courses interfered with. Some were turned out of their course, 
others were made to join different streams, many have been obliged 
to cut gorges, and a very large number have had their channels 
choked with glacial deposits, so that they have become locally trans- 
formed to lakes. The effects of the glacial accident will be better 
understood when we have studied glaciers (Chap. XVII). 

There are other less important accidents to which rivers are sub- 
ject. Sometimes a lava flow enters a valley and dams the stream 
back, forming a lake and changing its course, at times causing it 
to cut an entirely new valley, perhaps to join a new river system, 
thus rejuvenating the river by giving it a new task to perform. An 
avalanche from a mountain may produce the same effect, and the 
blowing of sand into the form of sand-hills sometimes dams a river, 
forming a lake. 

By one or all of these accidents many rivers are con- 
stantly being retarded in their development; and hence 
the task of river-valley formation is not only naturally a 
slow one, but one beset by many obstacles. Indeed, vari- 
ous portions of a single river may suffer different accidents 
and become composite in form. Again and again a river 
may start its development only to be interrupted; and 
although there are times when the accidents help the 
work along, on the average they give the stream some- 
thing more to do, and therefore prevent it from passing 
the stage of maturity into that of real old age. 

The River Course. — At first a stream chooses for its 
course the easiest slope, whatever this may be. This 
consequent stream course differs according to the location 
of the river. Upon a plain it may be very irregular, but 
in general will follow the direction of the slope of the 
plain, as the rivers of Florida flow outward from the 



RIVER VALLEYS 



279 



higher central part of the state, or as the streams of east- 
ern New Jersey and Texas flow outward toward the sea. 




Fig. 147. 

Map of a mountainous country, showing rivers parallel to ranges, and other 
features of drainage. 

Upon a country that has been glaciated, the surface is so 
irregular that streams are often obliged to pursue very 
roundabout courses 
before entering the 
sea. Among moun- 
tains, folded as 
they are into ridges 
and valleys, the 
larger streams flow 
parallel to the 
ridges, with tribu- 
taries running 
straight down the 
mountain sides, 
and the main 
stream now and 
then turning abruptly from its valley between the ridges, 




Fig. 148. 

River drainage on a plain, showing rectangular 

tributaries. 



280 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



and crossing a ridge to enter another valley, which it fol- 
lows for awhile (Fig. 147). Uj)on a plain the tribu- 
taries, which form an arborescent, interlocking series, 
may at first enter the main stream at right angles, or 
nearly so (Fig. 148); but as the work proceeds, they 
enter it at a more and more acute angle (Fig. 149), 
for they eat against the downstream bank, because the 
current is turned that way by the velocity of the water 
in the main stream. 

This consequent course may be followed for a time, but 
in many cases this course changes as the stream develops. 
If at first it was irregular, and a longer journey was 
undertaken than necessary, the river slowly straightens 
its course so as to flow more directly. Or if after cutting 

through a plane of horizontal 
rocks, the river finds itself cut- 
ting into tilted beds, it may 
find it necessary to adjust its 
course to agree with the tilted 
rocks. Such a river, which is 
called superimposed^ after cut- 
ting through the horizontal beds, 
may find itself flowing across 
the edges of a series of tilted 
layers, some hard, others soft, 
whereas if its course were just a trifle different, it could 
follow a single layer of soft rock, and therefore have an 
easier task. In such a case the river gradually changes, 
until it becomes more in accord with the rock structure, 
and finally becomes adjusted to it. 

Once in a soft bed, the river tends to remain there; 
and as the stream develops, there are many changes in 




Fig. 149. 

Map showing interlocking trib- 
utaries of rivers draining a 
plain. 



niVMB VALLEYS 



281 



course, until finally it has found the easiest path. This 
adjustment of river courses is not easy to comprehend 
without going more fully into the question than we are 
able to do here, and so it may be merely stated as a fact, 
that in regions where hard and soft rocks occur together, 
the harder beds stand up as hills or ridges, while the softer 
ones are lowlands and hence valleys, not necessarily 
because rivers were first located there, but because in the 
course of time soft rocks wear away more rapidly than hard. 

^ \ OLD VALLEY /" ^>^:1/Ar 







Fig. 150. 

Diagram showing the condition in parts of the Appalachiams where old moun- 
tain tops have been changed to valleys. 



Hence a stream, perhaps originally smaller, located in a soft layer, 
will develop more rapidly than another in a harder layer, and gradu- 
ally will gain upon its neighbor, and perhaps in the end entirely rob 
it of its drainage area. The stream that is more favorably situated 
cuts rapidly, its tributaries have more slope, and hence more power, 
and slowly they push the divide back into the area of the less favor- 
ably situated stream. Thus there is a constant but slow migration of 
divides, for the streams on one side are usually more powerful than 
those on the other. Hence the smaller stream may grow larger, and 
the large river dwindle, until by slow changes an adjustment is 
reached, with the larger stream located in the more favorable situa- 
tion. By such changes as this, mountain valleys have been trans- 
formed to mountain tops, and mountain tops to valleys (Figs. 150 



282 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



LIMESTONE 
jCjCbSc Sk Sc 



SANDSTONE 



and 151). This is common in the Appalachians, where the rivers 
are well adjusted to the rock structure. During this slow change 
there is sometimes a case where the divide is forced so far back, that 
the headwaters of the neighboring stream are actually drawn off and 
carried into the robber's territory, and then one of the rivers is 
quickly reduced in size, while the other is enlarged by the capture. 

There is a constant battle for territory between neigh- 
boring streams, and those that have the greatest slope, or 
the softest rock, or the heaviest rainfall, will be the most 
successful in the battle. Therefore we must look upon 
the river valley as a thing ever changing, and the river 

system as a thing of 
activity and even of 
life, struggling to 
reach a definite end 
of valley develop- 
ment. The river 
history is complex, 
and the valley prob- 
ably composite; but 
it has a story to tell: 
it is not a dead 
thing, formed and 
then left to remain 
ever the same with- 
out change. Look- 
ing at the surface 
of the land, every 
one may read a part 




Fig. 151. 

Map and section of the Sequatchie valley in 
the southern Appalachians, a river adjusted 
to the rock structure, and flowing in a bed of 
limestone which is a part of an old mountain. 



of the story told by the river valleys ; and by a more care- 
ful study it is possible to decipher many of the stages of 
the previous history, which has been so briefly outlined here. 



'^BATTLEDORE I. 
^1 HOG ISLANDS 
LITTLE BIRD I. B R E T 2T 

S U N D 




MISSISSIPPI RITER 

FRO.M THE PASSES TO GRAND PRAIRIE 



Facing page 283. 



Plate 18. 
Delta of the Mississippi. 



BIVER VALLEYS 283 

River Deltas. — When the water of a river enters a 
quiet lake, or the sea, its velocity is checked ; and if it is 
carrying sediment, some of this must be deposited near 
the point of entrance. Hence near their mouths, rivers are 
dumping a load of rock fragments, sometimes coarse, 
sometimes fine. In this way deltas are built. They 
are flat-topped plains of alluvial material, extending be- 
neath the sea or lake, to their end, which is a steeply 
sloping embankment (Fig. 152). This steep seaward face, 
which is entirely submerged, grows outward as more and 

DELTA PLAIN 

-=-=^-=-=r-^=-=-^=.=___ SEA 









» >> >^ N-^^ N \\ \ N^Ct-x 



Fig. 152. 
Cross section of a delta, showing its structure in a diagrammatic way. 

more is added ; and it remains steep for the same reason 
that a railway embankment does when loads of gravel are 
dumped upon it. 

The reason why the top of the delta is a plain, is that its 
surface cannot be raised much above the level of the sea. 
It does rise slightly above sea level, because as the river 
flows out over the plain which it has built, it flows over 
such a moderate slope, that the channel is not able to hold 
all the water in flood times. Then the plain becomes 
transformed to a broad, lake-like expanse of slowly mov- 
ing and shallow water, in which sediment settles, gradu- 
ally building the plain higher, but never to any very great 
elevation above the sea or lake. 



284 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Because of the very levelness of the delta plain, the 
water of the stream that forms it often flows through sev- 
eral mouths or distributaries^ which divide from the stream 
near the head of the delta, spreading out fan-shaped. 
Hence the delta, instead of being formed by one chan- 
nel, is often made by several, and its front is broad, 
so that the form of the delta plain, between its sea 
margin and the two outer distributaries, is often trian- 
gular, like the Greek letter delta (A), whence its name. 
In the larger deltas of the world, streams flow in very 
uncertain courses, so that a slight cause is often sufficient 
to make the stream abandon one of its former channels. 
Such rivers as the Yellow of China change their course 
frequently, flooding farms and villages and often destroy- 
ing much life. This is because the slope is so slight that 
the river deposits sediment, and builds its bed higher, so 
that in time it abandons its old course to find a new and 
lower channel. 

Deltas are not always, nor in fact usually, found at 
stream mouths. They are much more common in lakes 
than in the sea, mainly because lakes are shallow, so that 
less sediment is needed to raise the bed to the surface of 
the water. The fact that the depth is great is one of the 
reasons why deltas are absent from most river mouths on 
the seashore ; but one may be certain that any stream 
which carries sediment would in time build a delta, unless 
there were some other cause which prevented ; for no matter 
how slow the accumulation might be, year by year it would 
rise nearer the surface, until finally it reached sea level. 

Among these interfering causes are the presence of 
strong waves^ tidal currents, or other movements of the sea, 
which take the sediment as fast as it comes, and distribute 



mVEB VALLEYS 



285 



it far and wide. This is one reason why, next to lakes, 
enclosed and nearly tideless seas, such as the Mediter- 
ranean, are the places where deltas abound. Of course a 
river with much sediment will ordinarily build a delta 
faster than one carrying little or none ; but there are 
cases of rivers heavily laden with sediment and entering 
enclosed bays, yet not constructing deltas. Such cases 




Fig. 153- 
Alluvial fans in the arid lands of the west. 

are those in which the bed of. the sea is sinJciyig, or has 
recently sunk so rapidly that the river deposit has not 
been able to build up to the sea level. For delta forma- 
tion the most favorable conditions are much sediment, 
absence of strong waves and currents, a sea not too deep, 
and a sea bottom either remaining in the same position or 
else being slowly elevated. The absence of the latter 
condition explains the absence of deltas on the coast of 
eastern North America and western Europe, where the 
land has recently subsided. 



286 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Even on the land, rivers sometimes make a deposit which some- 
what resembles a delta. Where the stream comes down from a steep 
mountain valley upon a plain, its velocity is checked almost as effectu- 
ally as if it had entered the sea ; and if it is bearing much sediment, 
as it often is, some of this must be deposited in the form of a fan or 
cone-shaped accumulation, with the apex at the point where the stream 
emerges from the mountain. Over this alluvial fan, fan delta, or coi^e 
delta (Fig. 153), the stream flows by means of distributaries, con- 
stantly adding to its height and extending its area. Like a delta 
this deposit is somewhat triangular in outline ; but it has not the 
flat surface nor the steep front of the delta, but has a gradual slope 
from apex to base. 

River Floodplains. — Streams that are carrying much sediment are 
often obliged to deposit some of it in the channel, in places where for 
any reason the current is checked. This may happen when the floods 
subside; or during ordinary times the condition may occur in an 
eddy in the current, or on the down-stream side of a boulder, or of a 
tree that has become lodged in the channel. The bar may be very 
tiny, or it may grow to the size of an island, which divides the river 
and is covered only in flood stages. Some island-like bars are caused 
by the splitting of the stream, which for awhile follows two chan- 
nels, forming an island in the middle ; but in time one of these is 
abandoned and one chosen as the channel for all the water. In 
almost any stream valley of moderate slope, one may see a great 
variety of bars, islands, and partly closed stream channels, and 
by the study of them one may often find their cause. Most rivers 
are constantly changing their position, and as they do this, the form 
of their channels. 

Oftentimes the river is bordered on one or both sides by a plain 
which in time of flood is covered by river water (Fig. 138). This 
^' river bottom," or Jioodplain, may be narrow, or, as in such large 
rivers as the Mississippi, very broad. Each time that the river floods 
overspread tlie plain, a tiny layer of sediment is deposited on its sur- 
face, and gradually it is built upward. Indeed, the floodplain is really 
made by the river floods, and represents the accumulation of sediment, 
^ where the river slope is not great enough for the flood water to carry 
all the load. Extensive floodplains are more common in mature rivers, 
and particularly near their mouths where the slope is less. After a 



RIVER VALLEYS 287 

delta is built, and the river flows out over it, it is transformed to a 
floodplain ; and v^hen a bay is filled with a level sheet of river sedi- 
ment, as Chesapeake Bay may some day be, this also becomes a flood- 
plain. 

Some of the narrow floodplains, especially those in mountain val- 
leys, are made of coarse gravel ; but the larger plains are built of very 
fine-grained clay, and they constitute some of the best farming land 
in the world. Many of the meadows on the sides of small streams 
are tiny, yet true, floodplains. On the larger plains the surface is 
nearly level excepting near the stream bank, where the elevation is 
slightly greater than on either side. The river channel on both 
sides is bordered by a low embankment or natural levee. This is the 
place where the rapid current of the channel and the current in the 
middle of the floodplain come in contact; and here the velocity of 
the water is less, so that more sediment is deposited, thus building 
the embankment. Upon these are built the artificial levees which 
men construct in order to confine the river to its channel and prevent 
it from flooding the neighboring plain. 

Over a broad floodplain the river flows with a curving or meander- 
ing course, circling about in great, swinging curves. These are con- 
stantly but usually slowly changing in form and position. In the 
meander the river current has its greatest velocity on one side (Fig. 
138) ; and hence in places it is cutting against the soft banks, thus 
increasing its swing as it eats its way into the land. This does not 
broaden the channel, but merely changes its position, for as the water 
cuts against one bank, it deposits sediment on the opposite one, where 
the current is not so rapid. Therefore the width of the channel 
remains about the same, but its position slowly changes, and in time 
the river wanders over all parts of its floodplain. On many of these 
level areas the river increases the curve of meander so that in passing 
down stream upon a steamer, one may often look across a narrow 
neck of land and see the river half a mile away, but several miles 
distant as the boat must go. Gradually eating against its banks, the 
river sometimes cuts through them, taking a shorter course and aban- 
doning the " ox-bow " curve, which then becomes a somewhat circular 
lake in the floodplain, called an ox-hoiv cut-off. These are common 
in streams and there are many large ones on the Mississippi flood- 
plain. 



288 



FIRST BOOK OF PHYSICAL GEOGBAPRT 



Waterfalls. — Waterfalls and rapids are formed where- 
ever the stream channel changes its slope rapidly. They 
are convexities in the river bed, and between falls and 
rapids there is no distinct difference, excepting that in 

the one case the bed 
descends with nearly 
vertical slope, and in 
the other less steeply. 
A fall may change to 
a rapid, or vice versa. 
There are various 
ways in which these 
conditions may be 
introduced into the 
stream bed. By some 
means the river may 
be turned out of its 
course and forced to 
fall over a precipice 
or down a steep hill- 
side. This was the 
case with Niagara, 
which at the close of 
the Glacial Period 
found its course leading it to the edge of the bluff at Lew- 
iston, over which it fell, forming the first Niagara, seven 
miles distant from the present fall. A lava flow, or a land- 
slide, or the growth of a mountain across a stream bed, 
may introduce a similar steep slope ; and sometimes rapids 
or falls are caused where a side stream brings coarse mate- 
rials into the main river, which, not being able to bear 
them away, allows them to accumulate, forming a steep 




Fig. 154. 

A view at Ludlow ville, central New York, dur- 
ing time of low water. A hard limestone 
bed with soft shales below causes a water- 
fall. Shales beneath cut out in the form of a 
cave. A miniature Niagara in all features. 



BIVER VALLEYS 



289 



slope in a part of the bed. There are rapids of this kind 
in the valley of the Colorado River. 

But many streams develop rapids and falls as they pro- 
ceed in the construction of their valleys. If in cutting 
down in their channels they encounter a hard layer with a 
soft one below, they can cut the latter more rapidly than 




Fig. 155. 
A general view of Niagara, showing Horseshoe Falls in the distance. 



the former, and hence increase their slope (Fig. 154). 
This very increase of slope gives them new power to dig, 
and so they deepen the channel more and more, perhaps 
in the end forming a very high fall. Many of the water- 
falls of New York, and other regions, have been formed in 
this way, and Niagara, though first caused as above stated, 



290 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



still continues to exist for this very reason. Therefore we 
may take Niagara as a type of this kind of fall. 

Niagara began as a waterfall at the bluff at Lewiston and 
has gradually retreated up stream, until it has reached its 
present position, where it is a great fall about 160 feet high 
(Fig. 155), and 7 miles from its original position, from 
which it is now separated by a gorge, 200 or 300 feet in 
depth, which it has cut out of the rock. The fall is still 




Fig. 156. 

A peat bog in the Adirondacks with a pond enclosed — the last remnant of the 
former lake. (Copyrighted, 1888, by S. R. Stoddard, Glens Falls, N.Y.) 

moving backward at a rate which has been measured. 
During the last 50 years Niagara has moved up stream 
about 250 feet, yet it stands at about the same elevation. 
The reason why Niagara has thus retreated is found in the 
difference in rock structure. Just beneath the water, at the 
crest of the Falls, is a sheet of hard limestone, beneath 
which are beds of soft shale. Dashing over the fall, the 
water digs into the shale and cuts it out from beneath the 



RIVER VALLEYS 291 

hard layer of limestone until some of this is undermined, 
when a block falls, and the waterfall moves up stream 
for a few feet (Fig. 154). 

This process of undermining continues, and Niagara 
remains as a fall because it cannot cut the hard limestone 
as fast as the shale. The fall will remain just so long as 
these conditions exist in the stream bed ; but it will of 
course disappear when the stream has reached its lowest 
slope, or the profile of equilibrium ; for it cannot then cut 
one part faster than another. There are thousands of falls 
in this country where the cause is the same as this; and 
these are all in streams that are young enough to be dig- 
ging into their beds, and hence able to discover differences 
in the hardness of the rocks. Therefore falls and gorges 
are closely associated. 

Lakes. — A waterfall is a convexity in the stream chan- 
nel ; a lake, a concavity. When for any reason a lake 
exists, the basin must be filled as high as the lowest part 
of the rim, where it will outflow, provided the rainfall is 
sufficient. There are many ways in which such basins 
may be caused. A surface of new land, like a sea bottom 
just elevated, may have saucer-like depressions upon it ; or 
these may be caused by the change of level of the land. 
For instance, a mountain developing across a stream 
channel, often causes a dam, behind which lakes gather ; 
or a lava flow may check a stream; or deposits left by 
glaciers may build dams. The latter cause accounts for 
most of the lakes of the w^orld, especially those of north- 
w^estern Europe and northeastern America (Chap. XVII). 
A lake is therefore a part of the river valley. 

Lakes will exist in the course of a stream only for a 
short time, because rivers "are the mortal enemies of 



292 FIRST BOOK OF PHYSICAL GEOGRAPHY 

lakes." They are cutting down the outlets and filling the 
basins with sediment. Generally they cut very slowly, 
because they have no tools with which to work, having 
been robbed of most of their sediment in passing through 
the quiet lake water, which acts as a filter. The lake will 
last until destroyed by the combined process of filling and 
cutting at the barrier, and the last stage will be a swamp^ 
for when the water becomes shallow enough, vegetation 
commences to grow, completing the filling, and transform- 
ing the water to land. The great majority of the swamps 
of the world are the result of the filling of lakes.^ When 
the lake is filled, the streams flow over the surface of the 
deposits, and being no longer robbed of their sediment, 
begin to cut into the lake beds, and perhaps cut canons 
where the lakes formerly existed. The filling of small 
ponds is a small task, but the destruction of such immense 
bodies as the Great Lakes is a much more difficult one, 
though still possible if time enough is allowed. 

Sometimes lakes are caused to disappear by other means. For 
instance, a great lake once existed near Salt Lake City, the Great Salt 
Lake being the shrunken remnant. This which once outflowed into 
the Columbia, was destroyed by a change of climate from moist to 
dry, so that the water evaporated faster than it was supplied. Also 
in the valley of the Red River of the North an immense lake has 
recently existed, at the time when the great glacier formed a dam and 
prevented the river from flowing northward, causing it to outflow 
toward the south into the Mississippi. This lake disappeared when 
the glacier withdrew far enough to allow the water to take its natural 
northward course. 

So also at a time when the glacier prevented them from outflowing 
as at present, the Great Lakes have been higher than now. For in- 

1 There are other kinds of swamps, the next most important being river 
swamps caused by river water overflowing its floodplain. 



BIVEB VALLEYS 293 

stance, once when the ice occupied the St. Lawrence valley, the lake 
waters rose higher than now, and Lake Ontario outflowed through the 
Mohawk. Even before this, when the Mohawk was also ice-filled, the 
lakes flowed through other channels, once past Chicago, and once past 
Fort Wayne, Indiana. While these high stages existed, the lake 
waters built beds, cut cliffs, and deposited layers of sediment over 
their beds, and these now appear on the land, so that we may study 
them, and from them see what is now being done in lakes. This is 
the reason for the extensive wheat plains of Dakota and Manitoba, in 
the valley of the Red River, which were built in the bed of a lake ; 
and also of the elevated beaches which pass through New York, near 
Lakes Ontario and Erie, and thence westward into Ohio. 

Lakes generally have outlets ; but sometimes, where the 
climate is dry, they do not fill their basins to the rim. 
There are manj^ such lakes in the Great Basin, and in 
other dry regions of the world, and among these are many 
salt lakes. In time any lake without outlet will become 
salt, because all water that is flowing over the rocks bears 
some salt. When it evaporates, the vapor is nearly pure 
water, without the salt, and hence the salt is left behind. 
So day by day, more and more salt is supplied, and the water 
that brought it does not flow away with it to the sea, but 
passes into the air without it. Thus little by little the 
fresh-water lakes become salt, and then Salter and Salter, 
turning to dead seas, and perhaps becoming so saline that 
some must be deposited as rock salt in the lake. Even the 
Great Lakes would become saline if the climate should 
become so dry that they could not rise to their rims. 



CHAPTER XVII 

GLACIERS AND THE GLACIAL PERIOD 

Valley Glaciers. — In some parts of the earth the snow 
remains on the ground throughout the year. This occurs 
on high mountain tops or else in high polar latitudes, 
where much of the precipitation is in the- form of snow, 
and where the effect of summer melting is not sufficient to 
remove the supply of snow. The line above which this re- 
mains permanently on the ground is the snow line. Among 
mountains the snow line, if present, is found in the upper 
portions ; and in temperate latitudes only the high valleys 
and peaks are covered with perpetual snow. Here, since 
each summer fails to remove the fall of the preceding 
winter, snow gathers year by year, filling many valleys 
and clothing mountain sides in a permanent coat. This is 
known as a snow fields and it is from here that valley 
glaciers, such as those of the Alps, have their origin. 

A snow field on a high mountain is elevated into the 
zone of strong winds ; and therefore much of the snow 
that falls upon it is whirled away, settling at some lower 
level, and very often in the valleys between the peaks. 
By this means there is a constant movement from the 
snow fields into the valleys. Besides this, the high peaks 
of the mountains are very steep, and much of the snow 
that falls cannot lodge upon the slopes, but slides down 

294 



GLACIERS AND THE GLACIAL PEEIOB 



295 



into the valleys. Much more slides down in the form of 
great avalanches^ or snow slides, after the snow has become 
so deep upon the slope that it must slip off. By this 
means the snow is prevented from reaching great depths 
in the high parts of the mountains, and much of it there- 
fore passes down into the valleys, as water does on the 




Fig. 157. 
Snow field in the high Alps. 



land. Indeed, we may call the snow field the supply region 
for the ice streams or glaciers which occupy the valley. 

In a snow field the material is true snow, in the glacier 
real ice. There is a region between these tvv^o, near the 
head of the glacier proper, w^here the snow is changing to 
ice, and this is called the neve. By melting and freezing, 
and by pressure, the snow becomes compacted into ice, 
and then it slowly flows down the valleys as glaciers, 
passing down by a slow movement, somewhat as wax will 



296 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



flow if a large piece is placed upon an inclined surface 
and gradually warmed. In other words, the glacier ice 
behaves like a viscous body. 

Hence supplied from the ice field, the valley glacier 
slowly passes down the mountain side, extending well 
beyond the snow field, just as a river flows out beyond 




Fig. 158. 
An Alpine glacier, showing snow field, ice stream, and medial moraine. 

the place from which its water comes. It will pass 
down as far as the supply exceeds the melting, and 
will then end : and the terminus of the glacier will there- 
fore extend much further if the supply is great, than 
it would if this were small. Throughout its passage 
down the mountain valley the glacier receives much rock 
material. Just as in the case of a mountain river, so here, 
weathering supplies rock fragments. Avalanches, single 



GLACIERS AND THE GLACIAL PERIOD 297 

blocks, and bits whirled by the wind, are carried upon 
the ice, forming a moraiyie. Those piles of rock frag- 
ments, dropped mainly from the valley sides, and resting 
on the surface of the glacier near its margin, are called 
lateral moraines. When two glaciers unite, two of the lat- 
eral moraines may join, forming a medial moraine (Fig. 158). 
This is a dark band of rock and gravel in the middle of 
the ice, and on some glaciers there are several of these. 




Fig. 159. 
Rough crevassed surface of Muir glacier, Alaska. 

Although when subjected to slow pressure the ice flows 
like a viscous body, if for any reason it is strained, or 
caused to move rapidly, it may crack, as we may break ice 
or wax by striking it a blow. Therefore when flowing 
over its bed, since this is generally an irregular rock sur- 
face, it is often cracked or crevassed, perhaps becoming 
exceedingly rough and even impassable. Where the val- 
ley bottom slopes rapidly, an ice fall may be caused, in 
which the ice is crevassed into an irregular surface, quite 
closely resembling a, river surface tossed about in a rapid. 



298 FIRST BOOK OF PHYSICAL GEOGRAPHY 

Through these crevasses some of the surface moraine 
falls to the bottom of the glacier, and it is then dragged 
along the bottom, where it obtains more material rasped 
from the bed. The rock fragments in the bottom of 
the ice form what is known as the ground moraine. 
This together with the lateral and medial moraines, 
journeys slowly forward in the glacier until the end is 
reached, where the ice melts and flows away in streams, 
while much of the ice load of rock materials remains 
behind, forming a moraine at the end of the glacier, the 
terminal moraine. If the ice front remains at one place 
for a long time, the terminal moraine may be built to a 
considerable height, being made of hills of gravel and 
boulders brought and dumped by the ice. 

As it passes over the rock of its bed, the valley glacier 
acts like a powerful sandpaper, grooving and polishing the 
surface over which it passes, and grinding the fragments 
to a fine clay. In its mode of ivorh it is unlike a river, 
for it presses down on its bed under the heavy weight of 
solid ice above, while water, buoying up the sediment which 
it carries, makes the pebbles and sand lighter. The gla- 
cier differs also in the material which it carries. Since the 
rock fragments are frozen into the ice, a large boulder is 
transported as easily as a bit of sand, and so they journey 
along side by side ; but in a stream the velocity may be 
rapid enough to carry sand, but not to carry pebbles. 
Hence it is that the deposits made by the glacier are 
composed of bits of rock varying from fine clay to large 
boulders, many of which are scratched because they have 
been ground under the ice (Fig. 162). But the materials 
deposited by rivers are assorted according to the size which 
can be carried with a given velocity.^ If the glacier dis- 



GLACIERS AND THE GLACIAL PERIOD 299 

appears from a valley by melting, — and many have done 
so in the past, — the moraines are left on the surface, per- 
haps damming the streams and forming lakes, or turning 
them out of their path into more irregular courses, in 
which perhaps they are obliged to cut new valleys. 

From the front of the glacier there generally emerges 
a stream (or sometimes several) coming from an ice cave, 
out of which they flow with considerable velocity, bearing 
much sediment, which usually makes them milky white in 
color. This they carry down their valleys, depositing 
some of it in the channel, when the slope decreases, or 
perhaps over floodplains, or in lakes. Therefore not all 
of the material which the glacier carries, remains in the 
terminal moraine at the margin of the ice. 

Valley glaciers move at a variable rate, depending upon 
the slope and the snow supply. Their movement is ordi- 
narily only a few inches or a few feet a day, but some of 
the valley tongues of ice on the Greenland coast move as 
much as 75 or 100 feet a day. The glacier movement is 
generally so slow that it is necessary to observe very care- 
fully in order to detect it. The movement is more rapid 
in the centre than on the sides, where it is retarded by 
friction, and for the same reason less rapid near the bottom 
than at the surface. They flow in valleys which pre- 
viously existed, and probably glaciers have never carved 
their valleys as rivers have ; but in some places they are 
widening and deepening them. Where they cut into the 
bed more rapidly than elsewhere, they have carved out 
basins in the rock, in which lakes later accumulate after 
the glaciers have left the valleys. 

Mountain valley glaciers exist in the Caucasus Mountains, the 
Alps, and in the Norwegian mountains, in Europe ; in New Zealand, 



300 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the southern Andes, and in various parts of central Asia and western 
America. There are many thousands in the world, but in the United 
States proper there are only a few small glaciers, in the northern 
Rockies and in the Sierra Nevada; but as soon as the Canadian 
border is reached they commence to be numerous among the moun- 
tains. Glaciers exist on the line of the Canadian Pacific Railway, 
and from this place to northern Alaska they are very numerous. No 
part of the world has larger or more perfect valley glaciers than 
Alaska. The most noted of these is the Muir Glacier, north of 
Sitka; but in this region, and further north in the Mt. St. Elias 
region, there are many other grand valley glaciers which frequently 
terminate in the sea. 

While glaciers are now scarce in this country, in recent 
geological times there have been many in the higher val- 
leys of the Rocky Mountains and the Sierra Nevada. 
These existed at the time of the Glacial Period, and have 
left a record of their existence in the presence of moraines, 
ice-polished rocks, and boulders which have been taken 
from the mountain tops down into the valleys. Where 
now there are only a very few tiny glaciers, scarcely more 
than mere snow fields, there were formerly hundreds of 
well-developed valley glaciers. Because of a change in 
climate they have gradually disappeared from the land. 

The Greenland Glacier. — In two parts of the world 
there are immense glaciers covering all the land and 
moving over both hill and valley. One of these is in the 
Antarctic region, surrounding the south pole, the other in 
Greenland, which is almost entirely ice covered. Almost 
nothing is known about the former, but many geologists 
have visited the latter. In Greenland all but the margin 
is buried beneath snow and ice, the total area of this great 
glacier being about 500,000 square miles, an area 10 times 
as great as the state of New York. Both in summer and 



GLACIERS AND THE GLACIAL PERIOD 301 

winter snow falls upon the high interior region, in which 
there is absolutely no land, and where the country has 
been buried to a great depth and its surface raised to an 
elevation of 10,000 feet by the accumulation of snow. 
The snow of the interior becomes compacted to ice at a 
slight depth, and as it accumulates, slowly flows out in all 
directions from the interior toward the coast, north, south, 
east, and west. It moves somewhat as a pile of wax might 
flow if a weight were placed upon the top. 




Fig. 160. 

Margin of Cornell Glacier, Greenland showing terminal moraine. Black 
layers of ice carry quantities of rock fragments. 

Near the sea there are occasional mountain peaks pro- 
jecting above ^he glacier, forming v hat the Greenlanders 
call a nunatak; and there are also many islands and penin- 
sulas over which the ice does not extend, though upon 
which there are many small valley glaciers. The great ice 
cap of Greenland, riding over all the land, and burying 
even high mountain peaks, moves slowly down to the sea, 



302 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



which it enters at the heads of bays or fjords, where it 
breaks off in the form of icebergs. Between these places 

the ice rests on the 
land (Fig. 160), where 
it melts, forming 
streams which course 
along between the 
land and glacier. Now 
and then the water is 
dammed into a tiny 
lake into which the 
streams carry much 
sand and clay, build- 
ing deltas (Fig. 161). 
Away from the 
coast the surface of 




Fig. 161. 



Delta in tiny lake fornaed by ice dam at mar- 
gin of Cornell glacier, Greenland. 



the Greenland ice c|ip is pure and free from moraine ; but 
near the sea, where peaks project above the surface, there 




Fig. 162. 
A scratched glacial pebble from moraine of Cornell glacier. 



GLACIERS AND THE GLACIAL PERIOD 



303 



are lines of moraine on the glacier, though these are not 
numerous. Therefore nearly the entire surface of the 
Greenland ice sheet bears no moraine ; but in the lower 
parts there is considerable rock material, consisting of 
fragments varying in size from grains of clay to huge 
boulders (Fig. 160). Therefore the glacier is armed with 
cutting tools with which it can grind the land over which 
it slowly glides. Indeed 
just beyond the edge of 
the ice are seen rock 
surfaces recently cov- 
ered, and now grooved 
and polished by the ice- 
scouring which they have 
received. That the ice 
worked well is shown 
by the scratches and 
grooves of the rock, 
which point in the direc- 
tion from which the gla- 
cier flows. These were 
formed by the ice, which 
pressed the boulders 
against the bed rock and 
dragged them along, as 
we might scratch two rocks by rubbing them together. 
The fi-agments brought in the ice are often quite unlike 
those on which the edge of the glacier rests ; and hence 
it is certain that they have been brought from some other 
place. 

Where the glacier enters the sea, the ground moraine 
materials float off in the icebergs ; but where it ends on 




Fig. 163. 
Photograph of a piece of floating ice, show- 
ing the relative amount of ice above the 
water surface (AA) to that below. 



304 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the land, some goes off in the streams, but much remains, 
building terminal-moraine ridges and hills, as in the case 
of valley glaciers. At some recent geological time the 
ice has been more extensive, having formerly covered much 
of the land which now rises above it, and perhaps having 
entirely obscured the Greenland margin. The evidence 
of this is the presence of moraines on the land, boulders 
that have been brought from some other place, and 
smoothed and scratched rock surfaces. In fact, these 
are the same as may now be seen exactly at the margin 
of the glacier. 




Fig. 164. 

Iceberg off the North Greenland coast. 

Icebergs. — When a glacier ends in the sea, fragments of ice break 
off and float away, forming icebergs ; and these vary in size from tiny 
pieces up to great masses, perhaps a mile in width, and 100, 200, or 
even 300 feet in height ; but in the Arctic, icebergs rising more than 
100 feet above the water are uncommon. Since ice when floating has 
8.7 parts below the surface to one above, a berg 100 feet high sinks 
deep in the water, and is really an immense ice mass. In the Antarc- 



GLACIERS AND THE GLACIAL PERIOD 



305 



tic, some huge icebergs have been reported to be so large that they 
have been mistaken for islands. 

Breaking off from the glacier front with a thundering crash, sound- 
ing like a volley of artillery, they rock backward and forward, setting 
the water into commotion and causing powerful waves to move out 
in all directions. Then they float majestically away, being driven to 
some extent by the wind, but mainly by the ocean currents, changing 
position now and then, and once in a while running upon a shoal, 




^^. 



^\1 'F< 












-h.---'^ ; ' \ 






H- 



Fig. 165. 

Map showing extension of ice in eastern United States (by shading). The 
heavy hnes mark the position of terminal moraines. 

where they remain until by melting they can once more float away. 
Gradually they journey toward warmer latitudes, slowly melting until 
they finally disappear, perhaps thousands of miles from their place of 
birth. The ocean steamers which cross the Atlantic to Europe, 
often encounter icebergs 1000 or 2000 miles from the parent glacier. 
Starting very often with a load of rock fragments in their bases, 
they strew these over the sea bed as they slowly melt. 

Glacial Period : Evidence of this. — Over northwestern 
Europe and northeastern America, down to the line marked 
on the accompanying map (Fig. 165), the surface presents 



306 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



many peculiar appearances. The soil is not the residual 
soil of rock decay that commonly forms when rocks are 
exposed to the weather, but generally consists of a clay 
in which there are occasional, or in some cases numerous 
boulders. This is known as boulder clay^ or till (Fig. 166). 
This till is not found everywhere, but here and there, 
especially in stream valleys, are beds of sand and gravel, 
not unlike those now occurring where streams flow from 
the end of a glacier, as in Greenland. These deposits 
are not strewn over the surface regularly, but in some 
cases are 200 or 300 feet deep, though elsewhere there 
may be no more than a mere veneer of boulder clay upon 
the rock. Indeed, in some cases the surface is bare rock. 

The pebbles and boulders which occur in the till are not 
the same as those of the rock in the neighboihood, but 

many have been 
brought from the 
north, some now 
found in the 
United States 
having come 
from Canada. 
Moreover they 
are scratched, 
quite like some 
now found in 
the Greenland 
moraines, as if 
they had been 
ground against 
other rocks ; and the bed rock of the region is scratched 
and polished as in Greenland (Fig. 167), and the surface is 




Fig. 166. 

Glacial till or boulder clay, Cape Ann, Mass. 



GLACIERS AND THE GLACIAL PEBIOD 



307 



scoured into rounded outline, giving the form known as 
roches moutonnees^ or sheep-back rocks. The grooves and 
scratches point toward the place from which the boulders 
have come, as they do in Greenland. 




Fig. 1G7. 
Rock surface in Iowa, scratched "by passage of glacier. 

In the regions where these peculiarities occur, there are 
also many lakes and waterfalls, where streams have been 
dammed or forced out of their valleys by deposits of boulder 
clay and gravel. There is an area extending across our 
country, in a general westerly direction, in which these 
conditions are found, but south of which they are absent. 
This line of division separates a land of lakes, falls, and 
gorges, or in other words of young streams, from one in 
which these are either absent or very much less abundant. 
Moreover, on the one side the soil is boulder clay, on the 
other residual ; and on the one side the boulders are for- 
eign to the region, and both the pebbles and bed rock are 
scratched, while on the other they are not scratched, and 
there are no foreign rocks. 

If we Ave re to go to this area of separation, we would find 
that it was marked by a line of hummocky hills quite 
closely resembling a terminal moraine. This, which wq 



308 FIRST BOOK OF PHYSICAL GEOGRAPHY 

may call the terminal moraine of the Glacial Period^ ex- 
tends across country, passing up hill and down ; and it 
marks the limit to which ice extended at a recent period, 
bringing boulder clay and scratching and grinding the 
rocks over which it passed. Because the conditions so 
closely resemble those seen at the margin of actual glaciers. 




Fig. 168. 
Terminal-moraine hills near Ithaca, N.Y. 

and because no other agent will do what has been done 
here, geologists have concluded that in a recent geological 
period, a great continental glacier covered northeastern Amer- 
ica and northwestern Europe, as Greenland is now covered. 

Cause of the Glacial Period. — Th^fact that the Glacial Period really 
has existed cannot be doubted; and this being so, we must believe 
that there has been a great change in climate which allowed ice to 
advance over the country, and then another which caused it to with- 
draw. This is in hg,rniony with the evidence concerning the shrink- 



GLACIEBS AND THE GLACIAL PERIOD 



309 



ing of the Greenland ice, and the disappearance of glaciers from the 
mountain valleys of the west. Concerning the cause of this there has 
been much discussion ; but like many scientific facts, the true expla- 
nation has not yet been found and proved. We have many theories, 
but it is difficult to prove one and disprove the others, for the question 
deals with conditions that are past, and the effects of which only can 
be studied. 



f'^-'^M^: 



Fig. 169. 
A boulder-strewn moraine hill, Cape Ann, Mass. 



Glacial Deposits. — When the ice front stood at any one 
place for a considerable length of time, it dragged ground 
moraine down to its edge and piled it up along the margin 
as a terminal moraine. There are many such moraines in 
the United States and Canada; for not only did the ice 
advance to and stand at the line described above, but as 
it slowly melted from the land, its front stood along lines 
further and further north, and each time that it halted a 
moraine was built. These terminal moraines are among 
the most characteristic land forms in this country (Figs. 



310 FIRST BOOK OF PHYSICAL GEOGRAPHY 

168 and 169). They consist of a dump of rock fragments, 
varying in size from boulders to clay, and their surface 
outline is irregular and hummocky, as any mass of earth 
would be if dumped without order. There are small hills, 
saucer-shaped valleys, ridges, and in general an exceed- 
ingly irregular surface. They are chiefly composed of till 
brought by the glacier and laid down without assortment, 
and hence without stratification; but they also contain 
beds of stratified sand and gravel, which were deposited 
by water furnished by the melting ice. Therefore these 
moraines are complex in structure as well as outline. 

When the ice advanced, it carried with it a load of rock 
fragments which it held as a ground moraine ; and when 
finally it disappeared, this was left as a sheet of boulder 
clay strewn over the surface, sometimes as a thin layer 
and sometimes in very thick beds. When the ice crossed 
valleys, till was often dragged down into these and left 
there, so that some valleys have been entirely buried, 
while others have a filling of perhaps 100 or 200 feet of 
boulder clay. Some of the till is very bouldery (Fig. 169) ; 
but in other cases, where the supply of hard rock was 
scanty, there are few large rocks (Fig. 168), and in some 
of the till no boulders are found. This till has various 
forms, but generally it is a sheet extending uniformly over 
the surface of the rock on which it rests. The streams 
from the ice have also deposited sheets of gravel and built 
low hills of various kinds, and the land which has been 
ice covered is therefore strewn with various kinds of 
glacial deposits. 

Effects of the Glacier. — While it existed the ice did 
much work. It has moved materials from one place to 
another ; it has swept off the old soil and replaced it by 



GLACIERS AND THE GLACIAL PERIOD 311 

a new kind ; it has worn and lowered many of the hill- 
tops, deepened some of the valleys, and partly or entirely 
filled others ; and it has left the land surface quite differ- 
ent from the way in which it was found. Still with all its 
work, it has not been able to erase the larger preglacial 
hills and valleys, but merely to modify them. 

The most important effect of glacial action has been 
upon the drainage. When the ice stood as a barrier on 
the land it stretched across many stream valleys whose 
slope was toward the north, and these were then trans- 
formed to lakes, Avhich being prevented from flowing as the 
land sloped, were forced to seek some other outlet. There- 
fore many north-flowing streams for awhile flowed south- 
ward, and some of these, cutting down the barriers at 
their outlet, lowered them so greatl}^ that when the ice 
disappeared, the slope of the land no longer led them 
northward, and they continued to flow in their reversed 
direction. 

One of the best cases of this is found in the headwaters of the 
Alleghany, which before the Glacial Period belonged to the St. Law- 
rence drainage, but now joins the Ohio. While these lakes existed, 
beaches and deltas were built in them; and since the water has now 
disappeared from the basins, we may often see these old lake deposits 
clinging to the hillsides. Not only did these conditions exist near 
the southern terminal moraine, but along the margin of the ice, where- 
ever it stood on the land. Hence as the glacier withdrew northward, 
as it was melting from the country, the zone of tempoiriry glacial lakes 
extended farther northward. 

Before the ice came, the land was carved into hills and 
valleys, and streams occupied the surface ; but while the 
glacier existed they were for a time extinguished. As 
soon as the ice left, the streams again occupied the land, 
but only to find the old valleys considerably altered. In 



312 FIBST BOOK m PBYStCAL GEOGRAPHY 

some cases, as in Ohio and the other level states of the 
plains, the valleys were entirely obliterated by glacial 
deposits, and new ones had to be started by young streams 
flowing on the plains of glacial deposit. In other cases, 
and particularly in the moderately hill}'- countries, streams 
were sometimes caused to leave their old valleys and carve 
new ones. Or they were turned from the old channels 
by glacial deposits and caused to cut gorges in the rock, 
or in the till, to one side of their former course. There- 
fore, they were locally rejuvenated ; and hence it is that 
so many streams in the glaciated country flow in gorges 
for a part of the distance, and that in these there are 
numerous waterfalls (Chapter XVI). 

With equal frequency dams of moraine have been thrown 
across the stream course, forming lakes. Upon the irregu- 
lar sheet of till, the terminal moraine and the gravel hills 
of United States and Canada, there are probably more than 
100,000 lakes and ponds. Practically all the lakes of 
northern United States and Canada are the result of 
glacial deposits, either because their surface was dotted 
with saucer-shaped depressions, or else because they have 
formed dams across stream valleys. This is one of the 
reasons for the existence of the Great Lakes, though they 
were undoubtedly caused in part by the action of ice in 
digging valleys deeper, and in part by changes in the level 
of the land which have formed rock dams. Even with 
these causes, much of the area of the Great Lakes is 
due to the effect of glacial deposits which have partly 
choked old preglacial valleys. 



CHAPTER XVIII 

SEA AND LAKE SHORES 

Difference between Lake and Sea Shores. — While there 
are many differences in detail, the shores of sea and lake 
so closely resemble one another that they may be con- 
sidered together. In both we find cliffs and beaches, 
promontories and bays; and in each there are many 
differences between these from point to point. In both 
places there are waves constantly at work, and these differ 
in force from place to place; and there are also wind- 
formed currents in each. But in several ways they differ : 
no great circulation, like that of the ocean currents, is 
found in lakes, nor are there well-developed tides. Be- 
sides these, animal life is less abundant in lakes than in 
the sea, and therefore certain kinds of shores which are 
constructed by ocean animals are never found in lakes. 
On the other hand, the action of plant life is different in 
the two bodies, being more important in the fresh water. 

Form of the Coast. — If we pass along an extensive 
stretch of coast line, we find many differences as we pro- 
ceed. For instance, the coast of New England, north of 
Cape Cod, is chiefly rocky, and exceedingly irregular. 
There are tens of thousands of islands and peninsulas, 
great and small, and as many bays, harbors, and estuaries. 
To go on foot along the margin of such a shore for a 

313 



814 FIRST BOOK OF PHYSICAL GEOGRAPHY 

distance of a dozen miles, as measured from headland to 
headland, may require one to walk not less than 100 
miles, in passing around the bays (Fig. 176 and Plate 1'^). 
While rocky in general, it would be found by such a jour- 
ney that the abrupt sea cliffs and headlands often enclose 
beaches, either of pebbles or sand, and that salt marshes 
and mud flats line the head of many of the bays. 

Passing south of Cape Cod, it is found that the coast 
becomes less abrupt and rocky, and at the same time less 
irregular, until on the Carolina coast the shore is a series 
of sand bars and beaches, with no rocks and iew irregu- 
larities, excepting those formed by the sand bars. Still 
further south, on the southern end of Florida, the coast 
again becomes irregular ; but here the islands, or as they 
are called, the ke^s, are made of coral fragments. Carry- 
ing the examination to more distant lands, w^e see that 
Avhile the coast of Europe and the northern coasts both of 
western and eastern America are exceedingly irregular, 
the west coast of South America, although rocky, is so 
uniformly straight that good harbors are scarce. These 
differences are due to perfectly natural causes, most of 
which can be simply explained. 

Sea Cliffs. — Beating against the coast, the waves, armed 
with pebbles and sand, are cutting into the land. They 
gradually wear away the hardest rocks, and on many 
coasts have made important changes, even since man has 
inhabited them. Where waves are at work violently, 
whether in lake or sea, their resistless sawing at the rocks 
cuts them into the form of cliffs (Fig. 170), which if the 
rock is hard, may rise nearly vertically, or if soft, with 
less steep slopes; for then the sand, clay, or gravel will 
slide down until it can come to rest. With the action 



M 



r m 






^ 




SEA AND LAKE SHOBES 



315 



of the waves and their allies, the wave and wind currents, 
not only is the rock cut away, but it is removed, leaving 
the cliff open to fresh attacks. Just so long as this is 
done the cliff will maintain its steepness ; for the waves 
saw along a narrow zone near sea level, and thus, nnder- 
cutting the cliff (Fig. 171), perhaps forming sea eaves, 
undermine the rock above, causing it to fall because its 




Fig. 170. 
A sea cliff on south shore of Bermuda. 

support is removed. So, gradually by this undermining 
action, fragments of the cliff face are caused to fall, and 
slowly it moves backward, the falling fragments being 
carried away by the waves. If the time comes when the 
materials cannot be taken away, the waves cease to cut 
into the cliff, and under the influence of weathering it 
gradually loses its steepness. 

Sea cliffs of great size, rising hundreds of feet out of 
the water, are found in the open ocean where the full 



S16 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



force of powerful waves can be exerted against the shore ; 
but similar though smaller cliffs occur in lakes (Fig. 172), 
and also in the enclosed bays of the seashore, where smaller 
waves are generated. Their form varies greatly with the 
kmd of rock out of which the coast is made ; some shores 
crumble so rapidly under w^eathering that they are never 
vertical, and this is particularly true where the material is 
soft sand or gravel. In other cases the rocks are so hard 
that they can be cut into the form of a cliff, which main- 




FiG. 171. 
Wave-cut islands, Bermuda, showing undercutting action of waves. 



tains the form of a precipice for a long time. Sometimes 
these vertical cliffs are several hundred feet in height, ris- 
ing directly from the water ; but in other cases the shore con- 
sists of rounded and somewhat irregular outlines. The sea 
cliff is the natural result of wave work where they are free 
to cut as they will. One may therefore expect that this 
would be not only the grandest but also the commonest 
of seashore features ; but the latter is certainly not true. 



SEA AND LAKE SHOBES 317 

The Beach. — When the waves are cutting against a 
cliff and wearing it back, they are obtaining materials 
which must be disposed of if they would continue their 
work of cutting. As has been explained, the materials 
wrested by the waves are removed by the undertow, the 
wind-formed currents, and the swash of the surf, which 
rushes upon the land at a slight angle to the direction 
in which the coast extends. By the first of these some 
of the material is carried off shore, and by the others alojig 
shore; but to that burden which the waves themselves 
take, is added one imposed upoii them. Weathering, wind 
action, and rivers, are furnishing other materials to the 
waves, and these supplies sometimes become so great that 
the waves are overburdened , and cannot perform the great 
task of removal thrust upon them. 

For instance, along the entire coast of the United States 
south of New York, excepting at the southern end of Flor- 
ida, the waves have more material than they can carry off. 
Hence it is that the great ocean waves that beat against 
the Carolina coast are not cutting cliffs in the soft rock, 
but are breaking upon sand beaches, often at distances 
of several miles from the real shore, from which the outer 
beach bars are separated by lagoons and marshes. These 
bars, such as those forming Cape Hatteras, have been built 
up by the waves out of materials which were furnished 
them, but which the}^ could not carry away. Being forced 
to lay down their burdens, they have built hundreds of 
miles of bars ; and then the winds, taking the sand from 
the beaches, have piled it up in the form of sand dunes or 
sand hills. Therefore the bars are partly wave-formed 
and partly wind-formed ; but the supply has been furnished 
chiefly by the rivers. 



318 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



Along an irregular rocky coast, like that of Maine, the 
waves have an easier task. They have harder rocks against 
w^hich to w^ork, and hence get less of a load to carry, and 
the rivers entering the sea there do not transport so much 
sediment. The coast is more irregular, and because of 
this the waves beat against the relatively few headlands, 
wear fragments off, carry some out to sea, and drive others 

along shore un- 
til an indenta- 
tion of the coast 
is reached, 
where the load 
is dropped, be- 
cause upon en- 
tering a bay or 
harbor, the 
power of the 
waves decreases. 
Hence upon this 
coast there are 
many little 
pocket beaches 
of sand, pebbles 
or boulders that have been wrested from the neighbor- 
ing cliffs and driven into the depressions. In the larger 
indentations there are extensive beaches of sand and peb- 
bles (Fig. 110). Further up, near the head of bays, where 
the waves are always tiny, mud flats exist, because here 
even clay cannot be carried away, and that which the 
waves wear off, as well as that furnished by weathering 
and streams, is accumulated there. 

By this means the bays and other indentations of the 




Fig. 172. 
Wave-cut cliff with beach in small hay, Lake Superior. 



SEA AND LAKE SHORES 319 

coast are gradually filling up. They become a dumping 
ground for the wave-derived materials, as well as those 
coming from the land itself. Because of this fact some 
harbors have been rendered useless, while upon others it 
is necessary to spend large sums of money every year, in 
order to keep them deep enough for large ships to enter. 
Therefore the double action of cutting the cliffs and fill- 
ing the bays results in production of mo7^e regular 




Fig. 173. 
Bar built across bay, Cape Breton Island, Nova Scotia. 

One of the first steps in harbor filling is the forming of a 
bar across the mouth of the indentation (Figs. 173 and 
174). Driven along shore, the materials are dropped, 
causing the water near the mouth of the bay or harbor 
to become shallower. Later as more is added, the bar 
reaches the surface, and finally stretches from side to side, 
perhaps completely enclosing it, and transforming it to 
a pond, though more commonly a small opening is main- 
tained through which the tide ebbs and flows. Along 



320 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



the United States coast there are thousands of such bar 
beaches, some of them miles in length. 

Wave-carved Shores. — Besides constructing these wave- 
huilt forms, and cutting the sea cliffs, the waves have done 
much work of carving rocky shores into irregular outline. 
Wherever there is a softer layer between harder w^alls, the 
waves will find this and eat into it more rapidly (Fig. 175). 
Hence a rocky coast on which the materials vary in kind, 




Fig. 174. 
Map of a part of Martha's Vineyard, showing arms of the sea cut off by bars. 

is liable to be very irregular, consisting of alternating 
headlands and minor indentations. But these do not 
become very great, because as they increase in depth, the 
waves that enter have less power, and they become then 
a place of deposit, and have beaches built at their heads, 
which protect the rocks from the further attack of the 
waves. 

It has sometimes been stated that such great bays as the 
Chesapeake, and the straits, bays, and harbors of the coast 
of New England, have been cut out by waves and tides; 



SUA AND LAKE SHOBES 



321 



but this is certainly not true, for waves cease to be able 
to cut when they enter bays, and tides have not the power 
to do the cutting. Indeed careful study shows that such 
bays are being filled, not deepened and enlarged by tides 
and waves ; and so it is necessary to look for another 
cause to explain these greater irregularities. 

A Sinking Coast. — The elevation of the land is sub- 
ject to frequent change, 
and some places are ris- 
ing, others sinking, while 
some have recently 
changed in one or the 
other of these directions. 
If the land should sink 
near the coast, one of the 
effects would be to cause 
the sea to enter the val- 
leys, transforming them to 
bays, if they were broad, 
or to fjords if narrow. 
Should the submergence 
continue, in time the water 
would rise over low di- 
vides and transform them 
to straits, and make some 
hills into islands, others into promontories and capes. The 
coast would then become very irregular (Figs. 176 and 
180, and Plate 19). 

This description of what would happen may be applied 
to the eastern coast of North America and the western 
shores of northern Europe. Here there are islands which 
resemble hilltops, straits between capes and islands, and 




Fig. 175. 

A wave-carved indentation on shore of 
Cape Ann, Mass. Small bay formed 
where a soft trap rock crosses the 
harder granite. 



\ 



322 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



bays, harbors, and fjords which end in river valleys on the 
land. In fact the resemblance is so close to what actually 
would happen if the land should be partly drowned, that 
it is one of the strong proofs that such sinking actually 

has occurred. For 
instance, the strait 
separating Great 
Britain from Europe 
appears to represent 
a low divide sunk 
beneath sea level. 
The Hudson River, 
into whicli the tide 
rises above Albany, 
appears to be a river 
valley carved on the 
land. The Bay of 
St. Lawrence and 
its tributaries, and 
the Chesapeake and 
its branches, appear 
to be nothing more 
than land valleys 
now partly beneath 
the sea (Fig. 146). 

A sinking coast 
brings the sea water 
in contact with the hard rock of the land, and therefore 
reduces the amount of material which waves can cut. It 
makes the shore line more irregular, and therefore fur- 
nishes depressions into which the waves can drop their load. 
It causes the water to rise higher and higher, submerging 




Fig. 176. 

Map of part of Connecticut, showing present 
outline of coast with bays, peninsulas, and 
islands, and (by shading) the similar outline 
which would result if the land should sink a 
hundred feet more. 



SEA AND LAKE SHORES 323 

the beaches which the waves commence to build. There- 
fore, for these various reasons, a sinking coast is liable to 
be not only an irregular one, but also one of numerous 
cliffs and few beaches. Thus on the coast of Greenland, 
which is now sinking, there are very few beaches; but 
high cliffs rise directly out of quite deep water. In lakes 
the rising of the water for any reason produces the same 
results, as may be seen on the south shore of the Great 
Lakes. 

A Rising Coast. — A rising coast is less irregular because 
the sea bottom is much smoother than the land. It is 
covered with soft mud and sand, and hence the waves 
obtain a great load, and generally more than they can 
carry off, so that bars are often built, as in the case of 
the coast of Texas, which has been recently elevated. The 
western coast of South America is also rising, and this is 
the reason why there are so few harbors and such a won- 
derfully straight coast. In lakes whose level is being 
lowered, similar though much less pronounced effects 
are produced. 

Marshes. — On many shores, such as those of eastern 
United States, there are extensive marshy plains in the 
protected bays and estuaries. These are seen on the New 
England coast as well as behind the sand bars of the more 
southern states. Salt marshes are formed by a partial, 
and in some cases a complete, filling up of these enclosed 
areas, which are out of reach of the ocean waves. Here 
rain and rivers wash materials into the arms of the sea, 
and to this deposit is added sediment driven in by the 
waves and borne by the tidal currents. Settling, the 
accumulation slowly raises the sea floor until the depth is 
shallow enough for certain plants to take root, and then 



324 FIRST BOOK OF PHYSICAL GEOGBAPHT 

these raise it still higher, partly by entangling sediment 
and causing it to settle, and partly by their death. 

The salt mai'sh grass can grow only in places where it is at some 
time exposed to the air, though it must also at some time receive a 
bath of salt water. Gradually the surface rises to the level of the 
high tide, becoming then a remarkably level plain, through which ex- 
tend numerous channels occupied by the rising tide, which fills them 
and then overflows the plain with a sheet of salt water. In time the 
marshes rise even above this level and then become dry land. Before 
this stage, in many countries, men have built dykes to keep out the 
salt water, and have established farms and even towns upon a plain 
which is really below the level of the high tide. Both in England 
and America there has been much land reclaimed in this way, and 
large areas of Holland were once salt marsh. 

The mangrove (Fig. 77), a tree which can grow with its roots in 
salt water, is building similar marshes, though in this case tree-cov- 
ered. In this country these are found in the bays of Florida, and 
they exist on other subtropical as well as tropical coasts. In lakes, 
similar treeless and tree-covered marshes are built by the aid of plant 
growth ; and many small ponds are bordered by such swamps, while 
some have been entirely replaced by them. Both in sea and lake 
these marsh lands can develop only where the waves are not active 
enough to remove the clay or sand in which the vegetation takes root. 

Coral Reefs. — Where conditions are favorable, animal 
life in the sea exists in great luxuriance ; and particularly 
is this true in the shallow water in and near the tropical 
regions, where the reef-building corals abound. These 
animals can thrive only where the water is warm and the 
tem23erature never lower than 68° or 70°, the depth not 
more than 150 feet, and where they are not exposed to 
the air between tides. In addition to this, the animals 
must have a good supply of food, and hence they are gen- 
erally found where ocean currents exist. If the water is 
muddy, or if fresh water enters the sea, they cannot 



SEA AND LAKE SHOBES 



325 



abound. Hence reef-building corals need a combination 
of unusually favorable conditions, and their distribution 
in abundant colonies is not great. Where these favorable 
conditions are combined, the abundance of coral and other 
lime-secreting animals is marvellous (Fig. 79). 

Corals may abound in the shallow waters near the land, 
along which they huildf ringmg reefs rising nearly to sea 
level. Or they may build a reef just off shore, which is 
then known as a harrier reef. They grow so luxuriantly 













■ 


I 


■Pi 


m 


SS^'^SS 


SSHH 


HJH^Hl 


i 


i 


^g 


Bi[ 


5^H!H?i~P«^BWiH 


■ 


i 



Fig. 177. 
Beach on coast of Florida. Position of coral reef shown hy line of hreakers. 

that they build the bottom up so near the surface that 
the waves, coming upon the shore, break over the reef, 
forming a great line of surf, in which the animals thrive 
because the water is kept in commotion, thus causing a 
constant passage of food, which must come to the corals, 
since they are firmly anchored in place. Upon passing 
over the reefs, the waves break off fragments of coral, or 
tear away entire masses, and drive them ashore upon the 
beach, where they are ground up (Fig. 177). 

This coral sand, taken by the wind, may be blown into 



326 FIB ST BOOK OF PHYSICAL GEOGBAPHT 

the form of hills; and hence land will be actually con- 
structed out of coral fragments. The southern end of 
Florida and the Bahama Islands are made of coral sub- 
stance ; and the entire area of the Bermudas, above sea 
level, is made of shell sand derived from reefs and built 
into the form of hills by the action of the wind. Along 
the shores of northern Australia there is a reef, called the 
Great Barrier Reef, which is over 1000 miles in length; 
and from this is being supplied a vast amount of material 
out of which rock is being built. 

In the southern Pacific there are many coral islands far away from 
any land, and having the form of a more or less perfect ring. These 
are known as atolls, and the coral ring rises a number of feet above the 
water surface, partly because the waves have thrown fragments above 
sea level, but chiefly because the wind has blown the coral sand from 
the beach into the form of low hills. ^ 

Islands. — Islands are either new land built up in the 
sea or else remnants of old land partly destroyed. The 
former may be called islands of construction^ the latter 
islands of destruction. By far the greater number of 
islands exist near the sea coast, but not a few are found 
in mid-ocean. In size they vary from tiny bits of land, 
covered at high tide, to immense islands, like that of 
Greenland. 

By Construction. — Islands maybe 5m?^ by various means. 
Near deltas they are often formed because the waves cannot 
remove all the sediment out to sea. Along such coasts as 
that of eastern United States, particularly along the shore 
line of the southern states, where the waves have more 

1 For the theories to account for these interesting atolls a book on 
geology may be consulted. 



SEA AND LAKE SHORES 



32T 



sediment than they can dispose of, the combined action of 
winds and waves builds a great many sand bars, which are 
true islands, generally very long and narrow. Along our 
coasts there are many thousands of these, mostly small, but 
sometimes a score or two of miles in length. Such bars 
can be made only 
near land, where 
the water is shal- 
low. Animals are 
also building 
islands ; the coral 
reefs and the 
atolls just de- 
scribed are of this 
class. Along the 
coast of Bermuda 
there are many 
tiny islands that 
have been built 
by serpula (a worm that makes a calcareous shell) into 
the true ring form of the atoll (Fig. 178). 

Islands may be constructed by the elevation of an irregu- 
lar sea bottom ; for then higher parts of the bed may be 
raised above the sea, forming islands. Or the folding of 
the sea bed through the formation of mountains may 
also raise parts above the sea level. This is the origin of 
many of the larger islands of the world. The East and 
West Indies are parts of mountains not now raised high 
enough to form a portion of the continents near which 
they lie. 

The Hawaiian Islands are an instance of this ; for 
between the several islands of this group there extends a 




Fig. 178. 
Serpula atolls, Bermuda. 



328 FIRST BOOK OF PHYSICAL GFOGltAPBT 

ridge of upfolded sea bottom, evidently a true mountain 
range beneath the sea. The peaks which rise from the 
crest of this ridge are illustrations of another type of con- 
structed islands, the volcanic. By outpouring of molten 
rock these peaks have been raised as islands, and in this 
case also there are many similar elevations not yet raised 
above the surface. All the isolated islands of the open 
ocean, excepting those made of coral, are volcanic peaks ; 
and it is probable that most, if not all of the isolated 




Fig. 179. 
Islands on shore of Bermuda, cut from the land (on right) by wave action. 

coral atolls of the mid-ocean have been built by animals 
upon platforms constructed by volcanic causes. 

Bi/ Destruction. — By far the greatest number of islands 
are caused by the destruction of land. For instance, the 
waves beating against the shore and wearing it back, may 
leave some parts standing for awhile, thus forming islands 
(Fig. 179). Such islands can never be large, being really 
tiny fragments of the coast line not yet destroyed. 

The sinking of the land accounts for the greatest number 
of instances of islands of destructive origin. The partial 
drowning of an irregular coast transforms the shore into 
an exceedingly irregular area of promontories and islands, 



S^A AND LAKE SHOBES 329 

the latter being produced when the hilltops are sur- 
rounded by water. The subsidence of the Bermudas has 
caused the large number of islands seen there (Fig. 180). 
Those extending along the coast of Maine are due to this 
cause also (Plate 19), and the vast number along the shores 
of northeastern America, some of which are of large size, 
represent merely high peaks of the old land now partly 
drowned in the sea. 




Fig. 180. 
Islands in the Bermudas, due to sinking of the land. 

Some islands are being increased in size by the causes 
which constructed them ; but as soon as these causes cease, 
they, like other islands, are attacked by the waves. Since 
they are surrounded on all sides by water they can be 
attacked in all directions, and this attack Avill be active 
on all sides, especially if they stand well out in the open 
ocean. Gradually then they disappear, and hence if new 



330 FIBST BOOK OF PHYSICAL GEOGBAPHT 

supplies be not added, any island will in time be destroyed. 
The volcanic peaks rising in the sea grow in size as long 
as the volcano erupts ; coral keys grow as long as the 
animals thrive ; and a sand bar, built by the waves, in- 
creases just as long as the waves bring more than they 
can carry away ; but when these conditions cease, as they 
may in time, the island is doomed to destruction. 




Fig. 18i. 
Island joined to mainland by two bars, Cape Breton, Nova Scotia. 

Promontories. — What has been said about islands applies to prom- 
ontories and to capes, which are merely small promontories. Some 
are built by the waves, others may be caused by the joining of coral 
reefs to the land, or by the elevation of the sea bottom, or by vol- 
canic eruption, or mountain growth. The Malay peninsula, standing- 
side by side with the island of Sumatra, is a mountain uplift just 
as is Sumatra itself. The Japanese islands, and the Philippines to 
the south of these, have been lifted out of the sea by mountain fold- 
ing ; and in time, if the uplift continues, they may be joined to the 
Karatchatka peninsula. 

Tiny capes may also be formed by wave erosion ; and small capes, 
as well as great peninsulas, like that of N^ova Scotia, or of Labrador, 
may be caused by the sinking of an irregular land. These represent 
the high places in the ancient country, and the sinking has not been 
great enough to transform them to islands, though by a very small 



SEA AND LAKE SHOEES 831 

additional submergence ISTova Scotia would be cut off from the main- 
land, just as Newfoundland now is. 

Islands may be transformed to peninsulas by means of bars built 
from them to the land by wave action (Fig. 181). On the coast of the 
United States there are many illustrations of this; and sometimes 
the connection is only made at low tide, while in other cases, the bar 
is so high that a road may be built upon it. 

Changes in Coast Lines. — In England the coast has 
changed very considerably in the past thousand years; 
and in America, during the short period of fifty years 
since good maps of the coast have been made, many 
changes have been noticed. These consist in a cutting 
back of the headlands in one place, the formation of bars 
in others, and the filling of bays in still other places. The 
seacoast is the seat of very active changes, otherwise this 
could not have been seen by man. 

Not only is there this action of the waves, but the out- 
line of coasts is slowly changing, either through rising or 
sinking (pages 321 and 323). There was a time when the 
New England land extended many miles further than 
now, and when present islands and capes were hilltops, 
and straits and bays were dry-land valleys between hills, 
quite like those now found in New England. 



CHAPTER XIX 

PLAINS, PLATEAUS, AND MOUNTAINS 

Plains. — The term plain refers to a rather level stretch 
of country of not very great elevation. It is generally 
somewhat irregular, being crossed by streams which have 
carved valleys, and its surface often consists of a series 
of wave-like undulations. A large plain presents the most 




Fig. 182. 
A view on the plain of the Everglades in southern Florida. 

monotonous scenery to be found in any part of the land, 
for as far as one may look there is nothing but level 
country. Where plains exist at a considerable elevation 
above the sea, they may be more deeply dissected by valleys ; 
but these higher plains are more properly called plateaus. 

332 



PLAINS, PLATEAUS, AND MOUNTAINS 333 

There are many different kinds of plains. Bordering the coast of 
Texas, and in fact most of the states south of Xew Jersey, there is a 
narrow strip of level land which is really a part of the old ocean bot- 
tom, now raised into the air, just as plains would be produced if the 
continental shelf were elevated. The levelness of Florida is of the 
same origin ; but the delta and floodplain of the Mississippi have been 
built by the deposit of sediment carried by the river. Salt-marsh 
plains have also been built up, and there are many level stretches 
where lakes have once existed. In JSTorth America many small plains 
and swamps are really old lakes; and the great plain of the Red River 
valley of the Xorth represents an old lake bottom. 

In addition to this, level country may be caused by denudation, for 
a land may actually be worn down until it is nearly level. The plains 
of the central states, probably never very high land, have been gradu- 
ally levelled by past denudation; and then much drift, left by the 
glaciers, has been deposited upon their surface, so that many parts 
have been filled, making the surface even more level than it was 
before the Glacial Period. Therefore the prairies of the Mississippi 
valley have a double cause for their levelness. 

Even without much denudation, plains may result if the rocks lie 
in nearly horizontal sheets, as they do in the greater part of this 
country. With the same climatic conditions over a large area, streams 
will cut through the rocks, forming valleys ; but between these there 
will be level areas, because the rocks lie in sheets which wear down 
with the same rapidity in all points, excepting where stream channels 
lie. 



Starting upon a plain, the rivers carve valleys, at first 
narrow and steep-sided, but in time becoming broader. 
If the elevation is not great, this does not decidedly 
roughen the surface ; but if the region is elevated, the 
valleys may^ become deep and the country hilly, until 
finally it loses the characteristics of a plain, as in the case 
of western West Virginia, Tennessee, Kentucky, etc. If 
time were allowed, the surface would gradually become 
smooth again, and the roughened plain would again become 



334 FIRST BOOK OF PHYSICAL GEOGRAPHY 

a true plain, which would be an old land ; but for the same 
reason that old river valleys do not exist, old plains are 
not found. 

Plateaus. — A plateau is an elevated plain (Fig. 145) ; 
but it also has other features. Being more elevated than 
a plain, the streams have more power to cut, and it is 

REMNANT 
OF PLAIN 

H 




Fig. 183. 
Diagram to illustrate dissection of plain where hard strata (H) resist denuda- 
tion, leaving rather flat hilltops between the river valleys. 

generally dissected by deep valleys, and even by canons. 
Therefore, although in places the plateau is level, one 
rarely has to go far to find it greatly roughened, as in the 
plateau through which the Colorado River of the west 
cuts its canon (Fig. 145). 

Plateaus are generally formed by the uplift of horizon- 
tal beds of rock which make the country level-topped, be- 
cause denudation, in carving them, finds hard layers, which 
being horizontal, resist denudation everywhere excepting 
where the streams cut through them (Fig. 183). As denu- 
dation proceeds, the plateau may become exceedingly irreg- 
ular and rough, as in the case of the Catskill Mountains, 
and the highlands of southern central and western New 
York, which are true, though much dissected plateaus. 
In the course of this denudation flat-topped areas may be 
left between the streams, and in the west the Spaniards 
have called these mesas^ or tables. If still smaller in area, 
and rising steeply as a hill, these remnants of formerly 
extensive level stretches are called huttes in the west 
(Fig. 129). 



PLAINS, PLATEAUS, AND MOUNTAINS 335 

Treeless Plains. — In this country most of the plains and plateaus 
are treeless, by far the greater number of these being so because of 
the dryness of the climate. This is true of the entire western plateau 
and most of the great plains west of the Mississippi ; but the cause of 
the prah'ies of the Mississippi valley must be different, because in this 
region the rainfall is quite heavy enough for tree growth. The ex- 
planation of this peculiar absence of trees is not entirely certain. 
Some have argued that the soil is too dense, but others believe that 
the Indians have caused the open prairie, — that they set fires in con- 
nection with their hunt for the buffalo, and have thus kept the land 
clear of trees. The latter seems to be the most acceptable explanation, 
for it is known that trees can grow in the prairie soil ; and moreover 
it has been proved that Indians did build fires, and that they have 
actually extended the area of the prairies since white men came into 
the region. Both on the plains and prairies, trees grow in the river 
bottoms near the streams. 

There are treeless plains in other regions: the salt marsh is an 
instance; and the low, level land along the Texas coast is without 
trees because it is too swampy for them to grow. The steppes of 
Russia and the pampas and llanos of South America, are great treeless 
plains. 

Mountains 

Nature of Mountains. — There is very much difference 
in the use of the term mountain, but to most it means any 
considerable elevation above the surrounding country. 
Therefore in a level region a small hill is called a moun- 
tain, and in mountainous countries high peaks are called 
hills. As used in this book, the term refers to parts of 
the earth's crust which have been uplifted as the result 
of folding or breaking of the rocks of the crust. That is, 
the strata have been folded into waves, as we might fold 
the leaves of this book. In plateaus the rock layers re- 
main nearly horizontal, as they were deposited ; but when 
raised into mountains they have been inclined, 



336 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



This folding of the rocks has occurred in various parts 
of the earth, the most notable cases in this country being 
the Appalachians in the east, and the several mountain 
ranges occupying the western part of the United States. 
In these places the rocks have been tilted, either by folding 
or faulting, so that they rise above the general level, 
sometimes to heights of 5000 or 10,000 feet. A series of 
such folds forms a system^ such as the Appalachians, which 
extend from Alabama to New York, or the Rockies, reach- 





Sections through portions of the Appalachians, showing folded rocks and faults. 
Former extension of strata indicated by dotted lines. Letters identify 
layers. 

ing from Mexico into Canada. Two or more systems 
closely associated, such as the Rockies, Basin Ranges, 
Sierra Nevada, and Coast Ranges, constitute a eordil- 
lera. 

In each system there are parts known as ranges^ which 
consist of uplifted portions side by side and separated by 
valleys ; and in each range there may be single ridges 
(Plate 20). The characteristic feature of each of these is 
that the rocks are tilted, so that the length of the elevation 
is greater than the width. But the most striking feature 
among high mountains is the peak (Fig. 185), a lofty 
elevation whose length and width do not greatly differ, 



PLAINS, PLATEAUS, AND FOUNTAINS 



337 



but which rises above the surrounding region, sometimes 
to a great height. It is the mountain peak which people 
have in mind when they name a hill, a mountain. Really 
these peaks are true hills of great size and height among 
mountains ; and it is not necessary that the rocks of which 
they are made shall be folded. 




Fig. 185. 
Grandfather Mountain ; a peak in the Blue Ridge of North Carolina. 

Development of a Mountain System. — In order to under- 
stand the origin of the features of mountains, perhaps the 
best way to proceed is to imagine that we can trace the 
growth of a mountain system. Let us suppose that it 
starts in the sea, where for some reason the bed is being 
raised and the layers of the bottom are being folded. 
Gradually the bed rises, with little other change than the 
folding (or perhaps faulting) and uplifting, though proba- 
bly volcanoes pour forth lava at various points along the 
crests of the rising ridges. Such a mountain range exists 
in the sea along the line where the Hawaiian Islands rise 



338 FIRST BOOK OF PHYSICAL GEOGRAPHY 

above the surface, and this range extends not less than 
1500 miles. 

As soon as the range rises into the air, a new chapter 
begins. Formerly, as the folds gradually rose, there was 
nothing to check the increasing elevation ; but as soon as 
they reach the air, the agents of denudation commence 
their work of sculpturing and destruction, rains fall, winds 
blow, rocks decay in the weather, rivers gather on the land, 
and perchance the ocean waves beat against the margin. 
So the folds no longer rise uninterruptedly, but their 
elevation is ineffectually opposed, and they continue to 
rise with battered and scarred surfaces, never reaching the 
height to which they would have risen had denudation 
been debarred. They begin to be sculptured, and the 
hard rocks stand up because the softer ones are worn 
away more rapidly. 

Such a stage has been reached by the Japanese Islands, 
which are even now rising mountains. They represent a 
mountain system with several ranges,^ but denudation has 
cut into the ranges, and finding hard layers of rock in the 
form of inclined sheets, has carved out ridges where the 
hard layers exist. Because the rocks were folded as sheets, 
the ridges are nearly parallel ; ^ and wherever denudation 
finds such hard layers, ridges may be formed, as has hap- 
pened in the Appalachians (Fig. 143), which are old moun- 
tains that have been exposed to the air for a long time. 
But if there exists a more massive rock, like granite, not 
standing in a layer, it also will resist denudation and stand 

1 As we may fold paper into two or three folds, each representing a 
range, and the whole a system. 

2 As our paper mountain would be if we should take a pair of shears 
and clip off the tops of the folds. 



PLAINS, PLATEAUS, AND MOUNTAINS B39 

up, thus forming not a ridge, but a peak (Fig. 185). A 
very large number of well-known peaks in the world ^ rise 
above the surrounding folds, because some such hard rock 
as granite has resisted denudation better than the softer 
ones that surround it. Had the durable rock existed as a 
la7/er, a ridge would have been produced, though perhaps 




Fig. 186. 
Mt. Moran, in Teton Mountains. 

one which has been carved into many peaks along the line 
of the ridge, as seen in the Teton Mountains of Wyoming, 
and others in the west. 

Carrying the development of the mountain system still 
further, it may rise and become a part of the continent, as 
the Japanese Islands will if they continue to be elevated, and 
as the Coast ranges of California already have. Between 
them and the mainland, there is at first a partly enclosed 

1 Such as the peaks of the White Mountains of New Hampshire, the 
Adirondacks, Pike's Peak, Mt. Everest in the Himalayas, the Matter- 
horn, and other Swiss peaks, as well as hundreds of others. 



340 FIRST BOOK OF PHYSICAL GEOGRAPHY 

sea, then as the outlet rises above sea level, great enclosed 
lakes outflowing to the sea, and then perhaps a great 
valley.^ Finally, as the elevation continues, the valley 
may become a plateau, or perhaps an enclosed basin, like 
the Great Basin between the Sierra Nevada and the 
Rockies, into which rivers flow without passing into the 
sea, because their water is evaporated by the dry air, which 
has been robbed of its moisture. 

By the growth of the mountains not only may great 
basins be enclosed between the systems, but other valleys 
of smaller size may form between the ranges, while rivers 
carve still others on the mountain sides, across the ranges 
(Fig. 143), and between the ridges. The valleys which 
the rivers carve are deep canons with steep and rocky 
sides, because the water courses down them with great 
velocity, and therefore can dig rajDidly (Fig. 134). 

The Destruction of Mountains. — Although mountains 
rise slowly, so long as they grow their elevation is usually 
more rapid than the downcutting by denudation ; but the 
streams have such a slope that their valleys are deeply 
cut, and differences of rock texture are brougnL into very 
sharp relief. It is not merely because of the slope of the 
streams, but also because the peaks rise into the higher 
regions of the atmosphere, where winds are fierce and frost 
action powerful, so that weathering is rapid (Fig. 127). 
Moreover, the high mountain tops rise above the timber 
line (Figs. 85 and 187), and are therefore not protected 
from the weather. Hence it is that a young, and particu- 
larly a growing mountain is carved into very rugged 
forms. Just as canons are characteristic of young streams, 

1 Like the Sacramento valley of California, which lies between the 
Coast Ranges and Sierra Nevada. 



PLAINS, PLATEAUS, AND MOUNTAINS 341 

SO rugged mountains, like the Rockies, Andes, Alps, and 
Himalayas, are also young. 

But in the course of time there comes a period when 
the mountain-building forces cease to cause the ridge to rise, 
and then denudation has full sway, and the ranges slowly 
melt down. The Appalachians, consisting of low ridges 
strikingly different from the Alpine ranges, have for a long 




Fig. 187. 
Near the timber line, Gallatin Range, Montana. 

time been subjected to destructive agents, until they have 
lost their ancient elevation and irregularity. In fact denu- 
dation may go further than this, and lofty ranges be re- 
duced to low hills. Nova Scotia, all of New England, and 
the very sites of the cities of New York, Philadelphia, Bal- 
timore, Washington, and Richmond, are all reduced moun- 
tains which once rose to heights rivalling those of the Rockies 
and the Alps. We know this because the rock layers are 
folded in such a way that if they were continued, as they 
once must have been, they would rise thousands of feet 
into the air (Fig. 184). Therefore, such is the length of 



342 FIRST BOOK OF PHYSICAL GEOGRAPHY 

time since the earth was young, and so powerful are the 
agents of denudation, that if they have plenty of time 
in which to work, even mountain ranges may be reduced 
to low hills. 

Other Kinds of Mountains. — While only elevations due to folding 
or faulting, or true mountain ranges, have been considered here, it 
must be said that there are true mountainous elevations which may 
be formed without rock folding. Thereat mountain peaks are caused 
by the sculpturing of rocks among ranges ; but wherever unusually 
high areas have been formed by the unequal carving of the strata, 
mountain peaks may result, even though not situated in regions of 
folded rock. Many of the buttes of the west are high hills or low 
peaks, and therefore properly called mountain peaks. 

Indeed, ranges of considerable extent may be formed where un- 
usually durable rocks have resisted destruction better than the neigh- 
boring land; and this is the case in the Catskills, where the strata 
are not folded, but where hard sandstone occurs, which is much more 
durable than the surrounding layers. They therefore stand higher, 
and have been carved into very irregular form, so that, though they 
are really a very much sculptured plateau, they closely resemble a 
true mountain range. 

The Cause of Mountains. — The origin of mountains 
leads us to a question about which very little is known. 
Since it is not possible to state even the most important 
suggested explanations, nor to discuss them fully, it will 
be well to state but one, the contraction theory^ which has 
found most favor, though it must be said that it has not 
been proved. We knoiv that the heat of the earth in- 
creases with the depth, and it is believed that the interior 
is highly heated. If this is so, the earth is cooling; for 
the heat is escaping into space just as certainly as it is 
from a stove into a room. A hot body is larger than it 
would be if cool, and therefore, as it cools, it becomes 
smaller. 



PLAINS, PLATEAUS, AND MOUNTAINS U^ 

So the theory is, that the earth, a hot body within, 
surrounded by a cold, solid outer crust, is cooling, and 
hence slowly shrinking. The interior is therefore con- 
stantly becoming smaller; but the solid crust, already 
cool, is not losing size, and therefore it must either be 
separated from the interior or else sink down upon it, 
as the core becomes smaller. If this is done, the only 
way in which it can fit the shrinking central part is by 
crumpling, as the skin of an apple does when this dries, 
losing water from the inner pulp and therefore becoming 
smaller. This supposed loss of bulk, or contraction of the 
earth's interior, is a very slow process, and therefore the 
uplifting of mountains will also be slow. The elevation 
of the crust will occur along lines which for some reason 
are weaker than other places. 

Really, contraction causes the earth's surface to slowly 
settle, and this sinking is evidently occurring over most of 
the sea bottom; but locally, along mountain^ ranges, and in 
the continents, portions are rising, for the crust has a 
greater diameter than the shrinking interior, which is ever 
becoming smaller; and hence, while the greater part sinks, 
some must rise.^ To the contraction theory there are some 
objections, though none that seem fatal to it as an explana- 
tion; and it now stands as the best attempt so far made to 
explain the fact, which all know so well, but whose cause 
is somewhere in the earth bej^ond the reach of human vision. 
Not being able to see and hence thoroughly understand 
what is going on below the surface, we can only reason 
upon the basis of the facts which we already possess. 

1 This can be proved experimentally by taking a ball and attempting 
to make a flannel cover, a little larger than the ball, fit upon its surface. 
In doing this there will be some ridges of cloth. 



CHAPTER XX 

VOLCANOES, EARTHQUAKES, AND GEYSERS 

Volcanoes 

Birth of a Volcano. — In the year 1831, in the Mediter- 
ranean south of Sicily, quite without warning, there rose 
out of the sea a great volume of steam, bearing in it red- 
hot cinders, which falling back, built a shoal in the sea, 
which shortly rose as an island. A volcano was born, and 
its birth was announced by a roar and commotion of the 
water. In a short time quiet again reigned, and in a few 
years Graham's Island had disappeared before the attack 
of the waves, and now no land exists to mark its site. 

The new island was low (being about 200 feet high, 
and 3 miles in circumference), and was built entirely of 
loose fragments of volcanic ash, light in weight because 
pierced by innumerable tiny cavities, quite like pumice. 
It had risen through the crust as liquid molten rock, 
driven upward by the steam, which expanding in the 
lava, had blown it full of holes.^ The steam, escaping 
as it does from an engine, carried the ash high into the 
air, until spreading out above, it was brought down by 
gravity, falling at one side of the vent through which it 
had escaped. This vent the steam kept clear, so that 

1 As steam rises through oatmeal, or as gas makes porous the bread 
that is becoming solid in baking. 

344 



VOLCANOES 345 

when the eruption ceased there was a cavity or crater 
there. Most of the ash, and especially the heavier pieces 
that settle more quickly, fell near the outlet on all sides 
of it, forming a cone^ which was nearly as steep as the 
angle at which loose fragments of rock will rest in the 
air.^ 

This newly born volcano, which died after a single gasp, 
illustrates perfectly a tyjDical volcano, though it rose only 
about 200 feet, while the elevation of most cones is 
measured in thousands of feet. Had there been another 
eruption, the size of the cone would have been increased, 
and in time a large volcano would have been built ; but 
during its history, perhaps with a life of tens of thousands 
of years, there would have been many changes, for volcanic 
action is very capricious. During long periods it might 
have been quiet, then perhaps a violent eruption might 
have partly destroyed the cone, sending it into the air; 
and first ash might have been erupted from the vent, and 
later lava. Also throughout its history, denudation, the 
enemy of the land, would have attacked it, removing some 
of the materials out of which it was built. To understand 
what might have happened during its history, let us look at 
what has happened to some of the volcanoes of the earth. 

Vesuvius. — When the Italian peninsula was first visited 
from the east, a lofty conical mountain rose above the Bay 
of Naples. As time passed, towns were built at its base 

1 An experiment illustrating this can be shown by having a U-shaped 
tube, containing sand, with one end rising through a piece of cardboard 
partly resting on a table. Upon blowing through the tube the sand rises 
in the air and builds a cone with a crater. Care must be taken not to put 
too much sand in the tube ; and in order to build a high cone, sand must 
be put into the tube several different times. 



346 FIBST BOOK OF PHYSICAL GEOGBAPHT 

and upon its sides. It had the form of a volcano with a 
crater in the centre, but it did not erupt. Apparently, 
therefore, it was a dead or extinct cone open only to de- 
struction from denudation. For centuries this condition 
lasted ; but the volcano was not extinct, it was only sleep- 
ing or dormant. About the year 79 a.d. the Bay of Naples 
was visited by earthquake shocks. Monte Somma, the 
ancestor of Vesuvius, was preparing for a terrific eruption, 
and during the year 79 an outbreak occurred, so violent, 
that when it ceased, the cone was changed in form. Part 
of the old rim of Monte Somma had disappeared, being 
blown into the air, while a new and smaller cone had been 
built amid the ruins. 

Monte Somma slept no more, and after a rest of many 
centuries the active Vesuvius was born. Ash rose thou- 
sands of feet in the air, and spreading out, formed a cloud 
so dense that the day became as dark as night. The ash 
fell upon the flanks of the mountains and upon the neigh- 
boring lowland. People fled before the shower of hot rock. 
Homes and towns were abandoned, and when the erup- 
tion had ceased, the sites of cities, villages, and farms were 
covered by a great barren stretch of pumice and ash. For 
centuries these were buried, and even now, no doubt, scores 
of towns are entombed beneath the products of this erup- 
tion. Two of them, Pompeii and Herculaneum, have been 
discovered and extensively excavated, showing the dwell- 
ings of the people who were driven from them more than 
eighteen centuries ago (Fig. 188). 

Since that terrible eruption of 79, Vesuvius has had 
periods of quiet ; but every now and then an outbreak has 
occurred, and the mountain is still, at the present day, an 
active volcano. There have been rests of many years, at 



VOLCANOES 



347 



other times frequent eruptions. Sometimes the outbreaks 
have been violent, again relatively quiet; but at no time 
has there been such a destructive explosion as that which 
gave modern Vesuvius its birth. Often the material 
erupted has been ash ; but at other times liquid rock has 
welled out, and flowing down the mountain side, has cooled 
to form solid lava. Therefore this volcano has sent out 




Fig. 188. 
A portion of Pompeii excavated from beneath the ash. Vesuvius in the back- 
ground. A part of the old rim of Monte Somma on the right. 

both ash and lava, so that it differs from Graham's Island, 
which had but one eruption, and that of ash. 

Krakatoa.— In the late summer of 1883, the sailors in 
the Straits of Sunda saw a great cloud rising above a small 
island, and this, upon spreading out, obscured the sun, 
while ash fell from the air. Upon the neighboring land 



348 FIEST BOOK OF PHYSICAL GEOGRAPHY 

the same was seen, and the ground was shaken, while upon 
the low coasts a great water wave rushed, destroying thou- 
sands of lives. Krakatoa, which had not been in eruption 
during this century, had again broken forth, with the most 
terrific explosion that man has recorded. Ash rose miles 
in the air, and spreading out, fell on the surrounding land 
and water, and for awhile it was so thick upon the surface 
of the sea, in the Straits of Sunda,^ that the progress of 




The volcano of Mauna Loa, Hawaiian Islands. 

vessels was impeded. So high did it rise that the lighter 
ash, floating about by the upper winds, staid suspended in 
the air for months, some of it falling in America and 
Europe. A great water wave, generated by the explosion, 
crossed the Pacific to the California coast, and it was ob- 
served on the shores of Africa and Australia. 

When the eruption had ceased it was found that Kraka- 
toa had been split into two parts, one of which had disap- 
peared into the air, leaving ocean water where there had 
1 For volcanic ash. and pumice will float in water. 



VOLCANOES 



349 



been dry land. The part of the island that remained was 
covered with a deep coating of ash, and not a living thing 
was left, neither plant nor animal. Since then there have 
been no more eruptions ; and now Krakatoa is either dor- 
mant or extinct, but which, cannot be told for centuries. 
This eruption may have been the death struggle of the 
once mighty cone, or it may have been but a temporary 
awakening, after a long rest. 




Fig. 190. 
The lava floor in the crater of Kilauea. 



The Hawaiian Volcanoes. — A series of eight islands lie 
in a chain in the mid-Pacific, and all of them have been 
built by volcanic eruptions. Most are now extinct and 
are rapidly disappearing before the attacks of wind, 
weather, rivers, and waves ; but upon the largest, Hawaii, 
there are several craters, one Kilauea, another Mauna Loa 
(Fig. 189), and a third Mauna Kea. The latter rises 
13,805 feet above the sea, Mauna Loa 13,675 feet, and 



350 



FIBST BOOK OF PHYSICAL GEOGBAPHY 



Kilauea about 9000 feet. The two latter are now active 
and have been well studied. 

One may ascend to the top of the immense crater of one 
of these and look down upon a lake of liquid rock, through 
which jets of steam rise, occasionally throwing bits of lava 
into the air. There is no danger in this journey, which is 

constantly being 
made by tourists, 
while a house is 
built for their ac- 
commodation on 
the margin of the 
Kilauea crater. 
Once in a while, 
however, on an 
average of once in 
about seven years, 
an eruption occurs ; 
but it is not at all 
like those just de- 
scribed. There is 
no ash, but a flow 
of lava, not from 
the crater, but out 
of the side of the 
cone, through 
which the molten rock escapes by a fissure which had 
broken open the side of the mountain. From this the 
lava flows down the mountain side, sometimes reaching 
the sea, and perhaps passing 30 or 40 miles before coming 
to an end. At first it flows rapidly, a glowing stream of 
liquid rock ; but soon, becoming cooled in the air, a crust 




Fig. 191. 

An eruption of a tiny volcano in the Mediter- 
ranean. 



VOLCANOES 351 

forms on the top, under which the molten lava is enclosed. 
Then it flows less rapidly and finally barely creeps, slowly 
advancing for weeks, until at last, when the force from 
behind is exhausted, it stops, perhaps at the very outskirts 
of some town which it has threatened to destroy. Nearly 
all the eruptions of these volcanoes have been of this 
nature, — flows of black lava, known as basalt. 

Other Volcanoes. — From the hundreds of cones in the world we 
might select other instances ; but these that have been given, furnish 
illustrations of the chief differences. Some, like Fusiyama in Japan, 
and many in the Andes, always erupt ash ; others, like the Hawaiian 
cones, practically always emit lava; but most, like Vesuvius, ^tna, 
and the volcanoes of Iceland, now erupt ash and now lava. Some, 
like the tiny volcanoes of the Lipari Islands in the Mediterranean, 
have toy eruptions, so moderate that they may be witnessed from a 
ship, near by, without danger ; and from these there is every gradation, 
to those terrible eruptions of Vesuvius and Krakatoa just described, 
and the frightfully destructive outbursts of the Icelandic volcanoes. 
While in some cases the outbreaks are frequent, in others they come 
at irregular intervals, perhaps centuries apart. Even the tiniest 
eruption is an impressive scene ; but a violent one is the most awe- 
inspiring phenomenon which nature presents upon the earth's surface. 

Materials Erupted. — In volume and importance, steam is the 
greatest of volcanic products. Often in the quiet eruptions of Kilauea, 
great banks of steam rise from the lava; and in great eruptions so 
much rises, that reaching thousands of feet into the air, it condenses 
into drops, and falling back to the ground, produces deluging rains, 
so heavy that torrents rush down the sides of the cone, adding to the 
destruction caused by the other products. Perchance falling upon 
loose ash, it washes this down with it in such quantities that a 
destructive torrent of mud passes on, overwhelming everything in its 
path. Herculaneum on the flanks of Vesuvius was buried beneath 
such a mudjiow. 

Other gases than steam arise, but they are of much less impor- 
tance. Sometimes the lava wells out quietly as a flow, and there is 
always lava in the tube of a volcano. However, when for any reason 



352 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the steam has the power, the lava is blown into shreds and bits, and 
upon cooling, forms ash and jmmice. These vary in size from mere 
bits of dust to huge blocks of rock. In nearly every case the ash is 
merely lava which the steam has blown out, though sometimes, when 
a mountain side like that of Krakatoa is blown into the air, it is 
apparently made up of the broken fragments of the mountain, like the 
bits of iron which are thrown out during a violent boiler explosion. 

Form of the Cone. — A typical volcano is a cone with 
a crater in the centre. Sometimes the perfection of this 
has been destroyed by a violent explosion, like that which 
split Krakatoa, and that of 79 A.D., which partly destroyed 
Monte Somma. Some cones are very low, if the eruptions 
have been few, while others are of great size. There is 
also a difference in the shape of the peak. For instance, 
Mauna Loa, though rising above the sea to the height of 
13,675 feet, does not rise steeply (Fig. 189). Its diameter 
at the base is great, and hence it is a moderately sloping, 
but very high cone. This is because the material of which 
it is built is lava, which flowing away from the place of 
exit, extends over a score or two of miles, first as a liquid, 
then as a more viscous body. Fusiyama in Japan, and 
many of the South American volcanoes, are narrow at the 
base but very steep. These are made of volcanic ash which 
has settled near the crater, taking an angle as steep as 
loose fragments can maintain in the air, just as a heap of 
dirt will when dumped from a wagon. Those that are 
made partly of ash and partly of lava will be less steep, 
and these are the most common of volcanoes, and are 
therefore more typical than either the steep Fusiyama or 
the great mound of Mauna Loa. 

There is another point, too : when a volcano erupts ash 
some is lost, for the winds carry it away ; but in the lava 



VOLCANOES 



353 



eruption all remains within a score or two of miles of 
the cone. Hence a much larger peak will be made by the 
same number of lava eruptions than of ash, provided the 
quantity sent out is the same. Therefore the hulk of 
rock in such a cone as Mauna Loa is many times as great 
as that of some of the ash emptors. 

Extinct Volcanoes. — Not only do volcanoes erupt, be- 
come dormant, and then active again, but sometime in its 
history every volcano will die, as certainly as every animal 



^^B^^^^ -sj^^r-?^-. -> •-'-- -->^^l 









Popocatapetl, Mexico. 



Fig. 192. 

A dormant or else extinct volcano rising above the 
plateau. 



and plant will. Of extinct volcanoes we have thousands 
of instances in the world, but in no part of the earth are 
they more numerous than among the Cordilleras of the 
western part of this country and Mexico. Some have 
ceased erupting for so short a period that it cannot be 
certainly stated that they are more than dormant (Fig. 
192). It need surprise no one to hear that a volcano in 
2a 



354 FIRST BOOK OF PHYSICAL GEOGRAPHY 

the west has again burst forth into activity. Near some 
there are lava flows that have certainly not been exposed 
to the air for a full century. 

Cones forming under the sea^ rise without being attacked 
by the forces of destruction ; but all through their history 
volcanoes that rise in the air are subject to the attack of 
the agents of denudation. The work of these is not so 
rapid as the supply of lava or ash, and so the cones rise ; 
but when these supplies are cut off, they slowly melt away. 




Fig. 193. 
Volcanic necks or plugs, remnants of volcanoes, on the plateau of New Mexico. 

Among the Hawaiian Islands, and elsewhere in the Pa- 
cific, there are cones in all stages of destruction, and the 
same is true among the plateaus and mountains of the 
west. First the crater is breached and gullied (Fig. 192), 
then the cone form disappears, and finally only the hard 
core of solid lava in the tube or neck stands up. All of 

1 It seems certain that there are active craters in places in the sea, par- 
ticularly on the Asiatic coast. 



VOLCANOES 



355 



the cone, the ash and the lava flows, have now disappeared. 
There are hundreds of these remnants in the west (Fig. 
193). 

In past ages volcanic cones existed in New England, especially in 
the Connecticut valley, and from there along the Atlantic coast at 
least as far as !N"orth Carolina. Some of the ancient lava flows still 
remain buried beneath other rocks, but the cones have long since dis- 
appeared ; and in the east there have been no volcanoes in times suffi- 
ciently recent to have left even a part of a cone. 




Fig. 194. 
Diagrammatic sketch to show (by dots) distribution of volcanoes. 

Distribution of Volcanoes. ^ — -At present volcanic cones 
are most abundant along the borders of the Pacific Ocean, 
though there are some elsewhere. In all cases they are 
either in or near the sea; and generally, if not always, 
they are associated with mountains that are growing. 
Those that are extinct, like the partly destroyed cones of 
the west, are found among mountains that have ceased 
rising ; and it seems certain that as soon as mountains 
cease their growth, the volcanoes that exist among them 



356 FIBST BOOK OF PHYSICAL GEOGRAPHY 

( 
die out. Very often, as in the Hawaiian and Japanese 

islands, the cones extend in chains along lines. 

Cause of Volcanoes. — Some action or condition in the earth melts 
the rocks in places. Some believe that the roots of volcanoes reach 
through the crust into the zone v^^here the heat is high enough to melt 
strata, and certainly there is such a zone, if the temperature of the 
earth increases at the same rate as it does in the part so far explored. 
Others think that the ynovement of rocks in mountain folding causes 
melting by the heat of friction of the particles moving over one 
another, as we may warm two pieces of rock by rubbing them to- 
gether. Since this mountain folding is probably due to the heat 
within the earth, in either case the Jirst cause for volcanoes is this 
heated condition ; though in the second explanation, it is thought 
that the melting is a secondary result of this. 

Whichever of these is correct, the expelling force of the lava is 
steam, so that this is the immediate cause for the eruption, as it is in 
the explosion of a boiler. The reason for the association of growing 
mountains with volcanoes, may be the melting of rocks, or it may be 
the squeezing of the melted rocks up to places where they can escape. 
Whichever is the true explanation, the folding of the strata causes 
breaks, or fissures and faults, through which the lava may escape. 
This accounts for their arrangement along lines. 

Explanation of the Differences in Volcanoes. — When the 
lava contains little steam, or is in such a condition as to 
allow it to readily escape, as it does from the lava lake of 
Kilauea, it cannot gain violence enough to blow the liquid 
rock into the form of volcanic ash ; but if for any reason 
its escape is retarded, it gathers force until escape is finally 
made possible.^ Sometimes a crust forms over the crater, 

1 It is very much like a boiler against the sides of which steam is con- 
stantly pressing ; but the boiler is strong enough to stand the pressure up 
to a certain limit, though after this is reached an explosion takes place. 
An engineer does not usually allow the pressure to become great enough 
for an explosion, but every now and then permits a little steam to escape, 
thus relieving the pressure. 



VOLCANOES 357 

or in the upper part of the neck, and this may grow so 
thick that the steam cannot blow it out, and then the 
volcano either becomes extinct or remains dormant, slowly 
gathering force enough for an outbreak. At times the 
plug becomes so strong, that when the pressure is high 
enough, it is easier to break a new opening than to force 
the plug out, as it is easier for some guns to explode than 
to drive the wadding out. Then a cone may be built at 
the side of, or near, the old one. Mt. Shasta is a double 
volcano of this origin. 

Earthquakes 

Before and during a volcanic eruption, the movement of 
the steam and the lava sends jars through the rocks, and 
the earth trembles, and sometimes is so violently shaken 
that buildings are thrown down. There may be hundreds 
of such earthquake shocks during a violent outburst, and 
the destruction of life is very great. Any jar to the rocks 
will produce an earthquake, if upon the land, or will cause 
a great water wave, if in the sea.^ Next to volcanic causes, 
the most important source of earthquakes is tlie breaking 
of rocks. As they move along the fault plane, jars pass 
out, and these reaching the surface, cause earthquakes. 
In many cases, as in Japan in 1891, the breaking of the 
rocks which caused the shock reached to the surface, and 
in this case the ground was cracked, roads rendered im- 
passable, and streams interfered with. Since both volca- 
noes and faults are associated with mountains, earthquake 
shocks are frequent only in those places. Hence the 

1 A miniature shock is caused when gunpowder is exploded, and the jars 
that pass over a frozen pond on a winter night are similar shocks. 



358 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



greater part of the earth is free from them, though near 
Vesuvius, on the western side of South America, in Japan, 
etc., earthquakes are exceedingly frequent. While slight 




Fig. 195. 

Effect of the Japanese earthquake of 1891 upon a railway bridge. The earth 
was shaken from beneath the track. 

jars are not uncommon in central and eastern United 
States, there have been only three violent earthquakes in 
these regions since they were settled; one during the last 
centur}^, near Boston, another in 1812, near the boundary 
of Louisiana and Arkansas, and one near Charleston, South 
Carolina, in 1888. During the same period there have 
been hundreds in Japan, or near Vesuvius. 

The earthquake, bemg a jar starting from some centre, extends 
outward from this in all directions, as a series of waves of rock move- 



EARTHQUAKES 



359 



ment, very much as the jar caused by a blow upon a piece of iron 
passes outward to both ends of the bar. If the shock started from a 
point, the waves would be spherical, and if the rock were of one kind, 
they would advance as spheres of movement, reaching points at equal 
distances from the place of origin, or focus, at exactly the same time. 
In reality the focus is not a point, and rocks diffe7' in character, so 
that the waves are much less simple. The place directly above the 
focus, called the epicentrum, is the place where the shock is most vio- 




FiG. 196. 

Diagram to illustrate passage of earthquake wave from a point, the focus (F) ; 
E, epicentrum ; I, isoseismals. 



lent, and its force decreases on all sides from this. At a certain dis- 
tance from the epicentrum, in all directions, the shock reaches the 
surface at exactly the same time. If the waves were really spherical, 
these places would be at equal distances from the epicentrum, but 
really they are not. The somewhat circular, though quite irregular, 
lines drawn around the epicentrum, and passing through places at 
which the shock was felt at the same time, are called isoseismal lines. 

To understand the effects of the earthquake a brief 
description of the shock which devastated Lisbon, Portu- 
gal, in 1755, may be introduced. The epicentrum was out 
to sea, not far from Lisbon. Without other warning than 
a sound like thunder proceeding from the ground, there 
came, almost at the same moment, a violent shaking of 
the earth which destroyed most of the houses in the city, 



360 



FIRST BOOK OF PHYSICAL GEOGRAPHY 



SO that in six minutes 60,000 people were killed; and fires, 
starting immediately after this, added to the destruction. 
The surface of the ocean fell, then rose, and a wave 
rushed upon the shore to the height of 50 feet above the 

ordinary level. Many 
people who had es- 
caped, gathered upon 
a quay or stone pier, 
which extended out 
into the water, where 
they would be free 
from the falling build- 
ings ; but this sank 
into the water, carry- 
ing the people with 
it, and hence Lisbon 
was nearly destroyed, 
while a large part of 
her inhabitants were 
exterminated. This 
was one of the great- 
est of earthquakes 
since man has kept 
definite records. The 
shock was felt in the 
Alps and in Germany, 
Sweden, and Great Britain, and the water wave reached 
across the Atlantic as far as the West Indies. 

When occurring in the open country, even a violent 
earthquake is not destructive ; but if the epicentrum is 
near a city, the falling buildings, and the fires that are 
caused, add greatly to the destruction. In countries like 




Fig. 197. 
Map near Charleston, S.C., showing the two 
epiceutra and the isoseismal lines during 
the shock of 1888. 



EARTHQUAKES 



361 



Japan, which are visited by frequent shocks, the inhabit- 
ants build low houses, so constructed that they are not 
easily thrown down ; yet even in Japan single shocks have 
destroyed thousands of lives. 




Fig. 198. 
Giant geyser in eruption, Yellowstone Park. 

Geysers. — Hot springs emit hot water, which flows out as it does 
from any spring, with the exception that the water is heated, perhaps 
even to the boiling point. In some cases the cause for this heat is 
no doubt intruded lava which has not reached the surface ; but at 
other times its cause may be friction along the sides of fault planes, 
where the grinding of the particles produced heat for the same reason 
that a knife becomes hot when held against a dry grindstone. Per- 
haps the water sometimes comes from such a great depth that it brings 
out some of the heat that exists deep in the crust. 

In several places in the world, but especially in the Yellowstone 
Park, New Zealand, and Iceland, the hot water comes forth not regu- 



362 FIRST BOOK OF PHYSICAL GEOGRAPHY 

larly, but intermittently, forming geysers. There are periods of quiet, 
and then, perhaps after intervals of a few minutes, hours, days, or 
even months, there comes an eruption, and a fountain of hot water 
and steam rises into the air and quickly subsides. Some geysers erupt 
at regular intervals, but others irregularly. The cause for the erup- 
tion is the presence of a supply of heat, which warms the water down 
in the tube until it reaches the boiling point, expands to steam, and 
expels the water above it into the air. Then being relieved, it re- 
mains quiet until the heat again raises the temperature of the water 
to the boiling point, when another eruption occurs. The time be- 
tween these outbursts will therefore depend upon the length of time 
needed to heat the water in the tube.^ 

1 A geyser may be artificially made by warming water in a long, nar- 
row glass tube, applying heat to one part of it. If the teacher wishes a 
more detailed explanation of the cause of geysers he will find it either in 
Le Conte's Elements of Geology, or in my Elementary Geology, p. 362. 



BOOKS OF REFERENCE 

Russell. Lakes of North America. Ginn & Co., Boston, Mass., 1895. 
$1.65. 

Russell. Glaciers of North America. Ginn & Co., Boston, Mass, 1897. 
$1.65. 

National Geographic Monographs, Vol. I. Articles by Powell, 
Russell, Shaler, Willis, Gilbert, Diller, Davis, and Hayes. American 
Book Co., New York, 1896. ."|2.50. Separate papers may be pur- 
chased singly. 

Merrill. Rocks, Rock- Weathering, and Soils. The Macmillan Co., 
New York, 1897. $4.00. 

Geikie. Ancient Volcanoes of Great Britain. The Macmillan Co., 
New York. 2 Vols. 1897. 

There are several articles of general interest in the annual reports of 
the United States Geological Survey, Washington, D.C. A list of these 
can be obtained from the Director. 



INDEX 



Absolute humidity, 127. 

Absorption of heat, 57 ; of light, 47. 

Abyssal fauna, 169. 

Accidents in river valleys, 27i. 

Adjustment of rivers, 281. 

Afterglow, 50. 

Age of earth, 236. 

Ages, geological, 238. 

Air, changes in, 42; composition of, 

33; effect of heat on, 64; height of, 

40; importance of, 7, 32; pressure 

of, 85 ; vapor in, 35. 
Alluvial fan, 286. 
Altitude, influence of, on climate, 163; 

on temperature range, 77 ; on v^eath- 

ering, 252. 
Andromeda nebula, 31. 
Anemometer, 101. 
Aneroid, 87. 
Animals, distribution of, 165 ; of land, 

178; of ocean, 167. 
Antarctic circle, 20. 
Antarctic zone, 67. 
Antecedent rivers, 276. 
Anticline, 236. 
Anticyclonic areas, 107. 
Anti-trades, 92. 
Arctic, animals of, 179; climate of, 

155. 
Arctic circle, 19. 
Arctic ice, 157, 190. 
Arctic zone, 67. 
Argon, 33. 
Artesian wells, 243. 
Ash, volcanic, 352. 
Asteroids, 23. 
Atlantic, currents of, 216; depth of, 

198. 



Atmosphere, 32; height of, 40; tem- 
perature of, 63. 
x^urora Borealis, 54. 
Avalanches, 259, 295. 



Bad Lands, 258. 

Barograph, 88. 

Barometer, 86. 

Barometric gradient, 89. 

Barrier reefs, 325. 

Barriers to spread of life, 185. 

Bars, 319. 

Base level of erosion, 270. 

Beach, 230, 317. 

Beds, 233. 

Belt of calms, 91, 150. 

Bermuda, animals and plants of, 183. 

Blizzard, 120. 

Boulder clay, 306. 

Butte, 253, 334. 



Calcite, 224. 

Carbonic acid gas, 33. 

Caverns, 246. 

Caves, 246. 

Centigrade scale, 69. 

Charleston earthquake, 358, 360. 

Chemically deposited rocks, 229. 

Chinook, 121. 

Circumpolar whirl, 94. 

Cirro-cumulus clouds j 139. 

Cirrus clouds, 138. 

Cleavage planes, 223. 

Climate, 149 ; influence on weathering, 

252; of ocean, 62, 
Climatic zones, 78, 149. 



363 



364 



INDEX 



Clouds, 135 ; cause of, 139 ; forms of, 
136 ; materials forming, 135. 

Coasts, changes in, 331; form of, 313; 
rising of, 323 ; sinking of, 321 ; wave- 
carved, 320. 

Cold pole, 80. 

Cold wave, 115, 160. 

Colors, cause of, 44, 47; sunrise, 49; 
sunset, 49. 

Columns, 248. 

Combustion, 33. 

Conduction of heat, 59, 61, 65. 

Cone deltas, 286. 

Cone, volcanic, 345 ; form of, 352. 

Conglomerates, 230. 

Consequent rivers, 278. 

Constructional islands, 326. 

Continental climate, 79. 

Continental glacier, 308. 

Continental shelf, 199. 

Continental slope, 199. 

Continents, 9; elevation of, 12. 

Contraction theory, 342. 

Convection caused by heat, 59, 65. 

Coquina, 231. 

Coral reefs, 168, 324. 

Cordilleras, 336. 

Coronas, 52. 

Crater, 345. 

Crevasses, 297. 

Crumpling of rocks, 233. 

Crust, condition of, 8, 220 ; minerals of, 
221; movements of, 234; rocks of, 
226. 

Crystalline rocks, 221. 

Cumulus clouds, 137. 

Currents in ocean, 215; of Atlantic, 
216. 

Cyclone, tropical, 116. 

Cyclonic areas, 107. 



Day, cause of, 13. 
Dead seas, 174, 293. 
Deflection of currents, 67. 
Degrees, 4, 5. 
Deltas, 283. 
Denudation, 260. 



Depths of the sea, 198. 

Deserts, cause of, 152. 

Dew, 130. 

Dew point, 127. 

Diathermanous bodies, 56. 

Dikes. 227, 233. 

Disintegration of rocks, 248. 

Dissected rivers, 276. 

Distributaries, 284. 

Distribution of animals and plants, 

182 ; of man, 180. 
Divide, 263. 
Doldrums, 91. 
Dormant volcanoes, 346. 
Drainage area, 262. 
Dredge, 194. 
Drowned rivers, 276. 
Dust particles, 38; effect upon heat 

rays, 66 ; influence on color, 46. 

E 

Earth, age of, 236; condition of, 7; 
form of, 3; interior of, 8; move- 
ment of, 13; origin of, 27; surface 
of, 9. 

Earthquakes, 357. 

Earth's crust, 8, 9, 220. 

Electricity, atmospheric, 52. 

Elements, 221. 

Epicentrum, 359. 

Equator, 6. 

Equatorial drift, 216. 

Equatorial zone, 67. 

Equinox, 16, 18. 

Erosion, 257 ; by rivers, 264, 

Ether, 22, 43. 

Evaporation, 25^62^ 126. 

Everglades, 33^. 

Extinct volcanoes, 346, 353. 



Fahrenheit scale, 68. 

Faults, 235. 

Feldspar, 223. 

Floe ice, 157, 190. 

Floodplains, 286. 

Focus of earthquake, 359. 

Foehn, 121. 



INDEX 



365 



Fog, 133. 

Folding of rocks, 234. 
Fossils, 238. 
Fragmental rocks, 229. 
Frigid zones, 78; climate of, 155. 
Fringing reefs, 325. 
Frost, 132; action of, in weathering, 
249. 

G 

Geological ages, 238. 

Geysers, 242, 361. 

Glacial deposits, 309. 

Glacial lakes, 311. 

Glacial period, 305. 

Glaciers, 294 ; erosion by, 257. 

Globigerina ooze, 203. 

Gneiss, 232. 

Graham's Island, 344. 

Granite, 227. 

Great Basin, 340. 

Greenland, climate of, 159; glaciers of, 

300. 
Ground moraine, 298. 
Gulf Stream, 217. 



Hail, 141. 

Halo, 51. 

Hawaiian volcanoes, 349. 

Haze, 134. 

Heat, absorption of, 57; conduction 
of, 59, 61, 65; convection caused by, 
59, 65; effect of, on highlands, 62; 
effect of, on land, 60, 64; effect of, 
on ocean, 61; latent, 62; nature of, 
55 ; radiation of, 57, 60 ; reflection 
of, 55, 61 ; from sun, 55 ; of evapora- 
tion, 62. 

Heat equator, 80. 

Heat lightning, 53. 

Hemispheres, 6, 10; laud and water, 
11. 

Herculaneum, destruction of, 346. 

Highlands, temperature of, 62, 79. 

High-pressure areas, 106, 107. 

Horse latitudes, 92. 

Hot springs, 242, 245. 



Humidity, 37, 127. 
Hurricanes, 116. 
Hygrometer, 129. 



Icebergs, 304. 
Ice fall, 297. 
Ice of Arctic, 157, 190. 
Igneous rocks, 226. 
India, climate of, 154. 
Insular climate, 78. 
Islands, 326 ; oceanic, 13. 
Isobaric, 103. 
Isoseismal lines, 359. 
Isothermal chart, 80. 
Isothermal lines, 79. 
Isotherms, 80. 



Kaolin, 224. 
Keys, 314. 
Kilauea, 349. 
Krakatoa, 347. 



Lakes, 291 ; formed by glaciers, 312. 

Lake shores, 313. 

Land, animals of, 178; effect of, on 
temperature, 72, 77 ; erosion of, 257 ; 
life on, 174; warming of, 60. 

Land breeze, 98. 

Land hemisphere, 11. 

Landslides, 259. 

Latent heat, 62. 

Lateral moraines, 297. 

Latitude, 5, 6; influence upon tem- 
perature range, 76. 

Lava, 351. 

Life, barriers to spread of, 185; in 
fresh water, 173; on the land, 174; 
in the ocean, 165 ; on ocean bottom, 
169, 191 ; zones of, 165. 

Light, absorption of, 47; nature of, 
43 ; reflection of, 44 ; refraction of, 
48 ; selective scattering of, 48 ; un- 
dulatory theory of, 43. 

Lightning, 52. 

Limestone caves, 246. 



366 



INDEX 



Lisbon earthquake, 359. 
Littoral faunas, 167. 
Longitude, 4, 5. 
Low latitude, 6. 
Low-pressure areas, 105, 107. 

M 

Magnetic pole, 4, 53. 

Magnetism, 53. 

Man, distribution of, 180. 

Mangrove swamps, 166, 324. 

Marshes, 323. 

Mature valleys, 270. 

Mauna Loa, 348, 349. 

Medial moraines, 297. 

Mercurial barometer, 87. 

Mercurial thermometer, 68. 

Meridians, 4. 

Mesas, 334. 

Metamorphic rocks, 231. 

Mid- Atlantic ridge, 200. 

Migration of divides, 281. 

Mineral springs, 244. 

Minerals of crust, 221. 

Mirage, 46. 

Mist, 135. 

Moisture in atmosphere, 126. 

Monocline, 236. 

Monsoons, 95, 

Monte Somma, 346. 

Moon, 24. 

Moraines, 297. 

Mountain breeze, 98. 

Mountain system, 336. 

Mountain valleys, 340. 

Mountains, cause for coolness on, 63; 
cause of, 342; climate of, 79; de- 
struction of, 340; development of, 
337; nature of, 335. 

Mountainous irregularities, 12. 

Mud flow, 351. 



N 

Natural bridge, 247. 
Natural levee, 287. 
Nebulae, 31. 
Nebular Hypothesis, 27. 



Neve', 295. 

Niagara, 289, 290. 

Night, cause of, 13. 

Nimbus clouds, 137. 

Nitrogen, 33. 

Northeast storms, 113. 

Norther, 120, 

Northern hemisphere, 6, 10. 

Northern lights, 54. 

North Polar zone, 67. 

North Pole, 4. 

North Temperate zone, 67. 

Nunataks, 301. 



Oblate spheroid, 6. 

Ocean, animals in, 167; area of, 187; 
currents of, 215 ; depth of, 12, 198 ; 
importance of, 7, 187 ; influence of, 
on climate, 163; influence of, upon 
temperature range, 72, 77 ; life in, 
165 ; movements of, 205 ; salt of, 
188 ; temperature of, 189 ; warming 
of, 61, 

Ocean basins, 9. 

Ocean bottom, animals of, 169, 191 ; 
materials of, 203; temperature of, 
195 ; topography of, 200, 

Oceanic climate, 78, 

Oceanic islands, 13, 

Old river valleys, 273. 

Opaque bodies, 47. 

Organic rocks, 229. 

Ox-bow cut-offs, 287. 

Oxidation, 33. 

Oxygen, 33. 



Peaks, 336, 
Pelagic faunas, 171, 
Periodical winds, 95. 
Plains, 332 ; treeless, 335. 
Planetary winds, 90. 
Planets, 23, 

Plants, distribution of, 165; of the 
land, 174 ; aid of, in weathering, 250. 
Plateaus, 334, 
Pocket beaches, 318, 



INDEX 



367 



Poles, 4. 

Pompeii, destruction of, 346. 

Pot holes, 266. 

Prairies, 335. 

Pressure of air, 85 ; change in, 

Prevailing westerlies, 93. 

Profile of equilibrium, 271. 

Promontories, 330. 

Psychrometer, 129. 

Pumice, 352. 



Quartz, 2!; 



Q 



Radiant energy, 44, 55. 

Radiation of heat, 57, 60 

Rain, 140; in cyclones, 113; distribu- 
tion of, 145 : erosion by, 258 ; meas- 
urement of, 144 ; nature of, 144. 

Rainbow, 51, 

Rainfall charts, 147. 

Rain gauge, 144. 

Ranges, 336. 

Red clay, 204. 

Reefs, 324. 

Reflection of light, 44 ; of heat, 55, 61. 

Refraction of light, 48. 

Rejuvenated rivers, 274. 

Relative humidity, 127. 

Residual soil, 257. 

Revived rivers, 274. 

Revolution, 15 ; effect of, on sun's heat, 
67. 

Ridges, 336. 

Right-hand deflection, 67. 

Rivers, course of^l^78 ; erosion by, 259, 
263. / 

River system, 262. 

River valleys, 261; accidents to, 274; 
effect of glaciers upon, 311 ; history 
of, 268. 

River work, 264. 

Rocks, of the crust, 226; disintegra- 
tion of, 248 ; igneous, 226, 351 ; met- 
amorphic, 231 ; position of, 232 ; sedi- 
mentary, 228 ; volcanic, 227. 

Rotation, 13; effect of, on sun's heat, 
66. 



S 

Sahara, temperature of, 73. 

Salt lakes, 174, 293. 

Salt marshes, 166, 323. 

Salt of ocean, 188. 

Satellites, 24. 

Saturation of air, 36, 126. 

Scattering of light rays, 48. 

Sea breeze, 97. 

Sea cliffs, 314. 

Sea ice, 157, 190. 

Sea level, 8. 

Sea shores, 313. 

Seasonal temperature change, 74. 

Seasons, cause of, 17, 21 ; effect of, on 

daily temperature change, 71. 
Sedimentary rocks, 228. 
Selective scattering, 48. 
Serpula atolls, 327. 
Shores, 313. 
Sirocco, 120. 
Snow, 141. 

Snowfall, distribution of, 148. 
Snow field, 294. 
Snow line, 294. 
Soil, formation of, 256. 
Solar system, 22 ; symmetry of, 27. 
Sounding machine, 193. 
South Pole, 4. 
South Polar zone, 67. 
South temperate zone, 67. 
Southern hemisphere, 6, 10. 
Space, 22. 

Springs, 242; mrneral-bearing, 244. 
Stalactites, 248. 
Stalagmites, 248. 
Stars, 26. 

Steam in volcano, 351. 
Stellar system, 26. 
Storm winds, 119. 
Storms, 102. 
Strata, 229, 234. 
Stratification, 233. 
Stratified rocks, 234. 
Strato-cumulus clouds, 138. 
Stratus clouds, 136. 
Submerged rivers, 276. 
Sun, 22 ; heat from, 55 ; position of, 15. 
Sunlight, measurement of, 52. 



368 



INDEX 



Sunrise colors, 49. 
Sunset colors, 49. 
Sunshine recorder, 52. 
Superimposed rivers, 280. 
Swamps, 292. 
Syncline, 236. 
System of mountains, 336. 

T 

Talus, 255. 

Temperate zones, 78 ; climate of, 160. 

Temj)erature in anticyclones, 115; in 

cyclones, 114 ; daily change of, 70 ; 

of earth's interior, 8; of earth's 

surface, 70; of ocean, 189, 215; of 

ocean bottom, 195 ; seasonal range 

of, 74 ; extremes, 84. 
Terminal moraines, 298; of glacial 

period, 308. 
Thermograph, 69. 
Thermometers, 68 ; dry and wet bulb, 

129. 
Thunder, 53 ; storms, 121. 
Tides, 210; cause of, 211; effects of, 

213; nature of, 210. 
Till, 306. 

Timber line, 176, 341. 
Time, reckoning of, 15. 
Time scale, 238. 

Topography of ocean bottom, 200. 
Tornadoes, 124. 
Torrid zone, 67. 
Trade-wind belt, 152. 
Trade winds, 91. 
Treeless plains, 335. 
Tropical cyclone, 116; zone, 67, 78; 

climate of, 150. 
Tropics, 19. 
Typhoons, 116. 



U 

Underground water, 242, 259. 

Undertow, 209. 

Undulatory theory of light, 43. 



United States, climate of, 161. 
Universe, 22, 25. 



Valley breeze, 98 ; glaciers, 294. 

Valleys in mountains, 340. 

Vapor, 35, 126 ; effect upon heat 
waves, 64. 

Vernal equinox, 18. 

Vesuvius, 345. 

Volcanic ash, 352. 

Volcanic necks, 354. 

Volcanic rocks, 227. 

Volcano, section of, 227, 233. 

Volcanoes, birth of, 344; cause of, 
356; differences in, 356; distribu- 
tion of, 355 ; of Hawaii, 349 ; mate- 
rials erupted from, 351 ; on moon, 25. 



W 

Water in earth, 240 ; effect of, on tem- 
perature, 72, 77 ; in volcano, 351 ; 
importance of, in weathering, 248. 

Waterfalls, 288. 

Water hemisphere, 11. 

Water parting, 263. 

Water vapor, 35. 

Wave-carved shores, 320. 

Waves, 205. 

Weather changes, 102 ; map, 102. 

Weathering, 248 ; effects of, 254. 

Wind vane, 101 ; waves, 205. 

Winds, 85 ; in anticyclones, 112, 119 ; 
in cyclones, 112, 119; erosion by, 
258; periodic, 95; planetary, 90; 
storm, 99, 112, 119; velocity of, 99. 



Year, 16. 

Young valleys, 269. 



Zones, climatic, 67, 78. 



Elementary Physical Geography* 



BY 



RALPH STOCKMAN TARR, B.S., F.G.S.A., 

Professor of Dynamic Geology a7id Physical Geography at Cornell University , 
Author of " Economic Geology of the United States," etc. 

Fifth Edition, Revised. i2nio. Cloth. $1.40 net. 



" There is an advanced and modernized phase of physical geography, how- 
ever, which the majority of the committee prefer to designate physiography, 
not because the name is important, but because it emphasizes a special and 
important phase of the subject and of its treatment. The scientific investi- 
gations of the last decade have made very important additions to the physio- 
graphic knowledge and methods of study. These are indeed so radical as 
to be properly regarded, perhaps, as revolutionary." 

"The majority of the Conference wish to impress upon the attention of the 
teachers the fact that there has been developed within the past decade a new 
and most important phase of the subject, and to urge that they hasten to 
acquaint themselves with it and bring it into the work of the school-room 
and of the field." — Report of Geography Conference to the Committee of Ten. 



The phenomenal rapidity with which Tarr's Elementary Physical Geography 
has been introduced into the best high schools of this country is a fact 
familiar to the school public. The reason should, by this time, be equally 
familiar — the existence of a field of school work in which, until the appearance 
of Tarr's book, there was not a single adequate or modern American text- 
book. That such a field did exist, is simply shown by the paragraphs reprinted 
above. The adoption of the book in such important high schools as those of 
Chicago, and the expressions of approval from representative New England 
schools, will indicate how well the field has been covered. 

The Physical Geography for Grammar Grades, by the same author, is 
almost ready. 

Tarr's High School Geology, uniform with Elementary Physical Geo- 
graphy, has attained wide use since its publication in February. 



THE MACMILLAN COMPANY. 

NEW YORK. CHICAGO. SAN FRANCISCO. 



Elementary Physical Geography, 

By PROF. RALPH S. TARR. 



From those who use it in New England Schools. 

Dr. F. E. Spaulding, Supt. of Schools, Ware, Mass. Tarr's Physical 
Geography has been in use in the Ware High School since September last. 
We regard it as incomparably superior to any other book on the subject. 
Previous to its publication, this most important and interesting department 
of science was seriously handicapped by the lack of a text suitable for use in 
secondary schools. Now no other subject taught in a high school can boast 
of a more adequate text than Tarr's Physical Geography. 

C. A. Byram, Principal, High School, Pittsfield, Mass. We have used 
Tarr's Physical Geography now for several months, and like it very much. It 
is both simple and scientific, while the make-up of the book is most pleasing. 

Miss H. A. Luddington, State Normal School, Fitchburg, Mass. I am 
very glad to express my great appreciation of the value of Tarr's Physical 
Geography. I rely upon it for clear statement and full illustration of all 
important topics in Physical Geography. As an aid to field-work in Physiog- 
raphy, I know of no book so helpful. 

Miss Maud L. Williams, High School, Northampton, Mass. I have never 
been so highly satisfied with any text-book as I am Avith Tarr's Physical 
Geography. 

Alfred 0. Tower, Principal, Lawrence Academy, Groton, Mass. I used 
Tarr's Physical Geography with my class last term, and consider it by far the 
best work published on this subject for High School or Academy use. 

J. C. Simpson, Superintendent of Schools, Portsmouth, N. H For the 
past year we have been using Tarr's Physical Geography in our high school, 
and as a text-book basis for an advanced study of geography by a class of 
teachers from our grammar grades. In both uses the book has given the 
highest degree of satisfaction. It seems to me to touch the ideal of modern 
geographic instruction more nearly than any other book published. 

Miss Atta L. Nutter, Miss Wheeler's School, Providence, P. I. Tarr's 
Physical Geography we are more and more pleased with as the work pro- 
gresses. 

G. W. Flint, Principal, High School, Collinsville, Ct. Tarr's Physical 
Geography has proved highly satisfactory in the class room to both pupils and 
teacher. The subject matter of the work and the arrangement and treatment 
make it the best text-book on Physical Geography that I have yet seen. 

F. A. Verplanck, Superintendent of Schools, So. Manchester, Ct. We use 
Tarr's Physical Geography with a class of Grammar School pupils and find 
it very satisfactory. I believe it is the best book on the market to-day. 



Elementary Physical Geography. 

BY 

RALPH STOCKMAN TARR, B.S., F.G.S.A., 

Professor of Dynamic Geology and Physical Geography at Cornell University. 
Fifth Edition, Revised. i2ino. Half Leather. $1.40. 



A PARTIAL LIST OF SCHOOLS USING THIS WORK. 

Normal School, Fitchburg . . . Mass. 

" " Framingham 

" " Salem 

" " North Adams 

High School, Amherst 

" ' " Bridgewater . 

" " Natick . 

" " North Attleboro 

" " Northampton 
" Pittsfield 

" " Springfield . 

" " Ware . 

" " Weymouth . 
Howe School, Billerica 
Lawrence Academy, Groton 
Training School, Holyoke . 
Tabor Academy, Marion . 
Worcester Academy, Worcester 
Morgan School, Clinton . . . Conn. 
High School, Collinsville . 
Wesleyan Univ., Middletown 
Normal School, New Britain 
Hillhouse High School, New Haven 
Williams Mem. Inst., New London 
High School, South Manchester 
East Greenwich Academy . . . R. I. 
Mr. Diman's School, Newport 
Miss Wheeler's School, Providence 
Normal School, Johnson . . . Vt. 
High School, Barre . . 

" " Brandon 

'* " Portsmouth . , . N. H. 
Wolfboro 
Seminary, Kent's Hill . . . Maine 

Teachers' College . . . Nevv York 
Columbia Grammar School 
Quincy School, Poughkeepsie 
Colgate Academy, Hamilton 
Manual Training School, 

Brooklyn 
Union School, Warsaw 
Academy, Middletown 

Stevens School, Hoboken . . . N. J. 
Collegiate Institute, York ... Pa. 
Columbia Academy, Washington . D. C. 
Public Schools, Xenia . . . Ohio 

" " Akron 

High School, Newark 
Prep. Dept. Univ., Wooster 
Webb School, Bell Buckle . . . Tenn. 
Citronelle College .... Ala. 
State Normal School .... Ind. 
High School, Westfield 

" " Marion . 
Peru . 

" " Pendleton 



High School, Attica . 






Ind 


South Bend 








" " Hanover . 






(( 


" " Connersville 






« 


" " Jackson 






Mich. 


State Normal School, Ypsilanti 






The Fourteen High Schools, Chicago 


111. 


including those at 




Hyde Park . . 


<( 


So. Chicago . 




«« 


Englewood . 




«< 


Morgan Park 




" 


Oak Park • 




" 


Aurora . 




« 


Lewis Institute, Chicago 




«* 


Armour Institute, Chicago . 
Acad, of N. W. Univ., Evanstor 




" 




(( 


High School, Carthage 




" 


" " Princeton 




(( 


Pittsfield 




<« 


" " Waukegan 




<« 


Lake Forest Academy 




" 


Morgan Park Academy, Morgar 


iPar 


< " 


Monmouth College, Monmouth 




<< 


High School, Kansas City . 




Mo. 


" " Hannibal 






" " Burlington . 




la. 


" Cedar Falls . 






" " Davenport . 




" 


" " Marshalltown 




<< 


" " Iowa City 




«« 


" " Grand Rapids 




Wis. 


Randolph 




" 


" Marshall 




Minn. 


Shattuck School, Faribault . 






State Normal School, Winona 




<« 


High School, Lincoln 




. Neb. 


Agricultural College . 




N. D. 


Normal School, Los Angeles 




Cal. 


" " San Jose 




" 


" " Chico . 






" 


High School, Riverside 






•' 


" " Sacramento 






" 


" " Stockton 






'( 


St. Helena 






" 


Redlands 






«' 


" " Petaluma 






" 


" Visalia . 






<< 


Selma . 






" 


Tulare . 






<< 


Univ. of Pacific, College Park 




" 


Gartner Seminary, Irvington 




'• 


Los Angeles Academy 




" 


Throop Polytechnic Inst., Pasadena 


" 


Trinity School, San Francis 


>co 




'• 



ELEMENTARY GEOLOGY/^/"^ 

G- , 

RALPH STOCKMAN TARR, B.S., F.G.S.A., 

Professor of Dynamic Geology and Physical Geography at Cornell University; 
Author of ^'Economic Geology of the United States," etc. 



i2mo. Cloth. 486 pp. Price $1.40 net. 



COMMENTS OF THE PRESS. 

^'We do not remember to have noted a text-book of geology which 
seems to so go to the heart of the matter." — Phila. Evening Bulletin. 

"The author's style is clear, direct, and attractive. In short, he has 
done his work so well that we do not see how it could have been dope 
better." — Journal of Pedagogy. 

" It is far in advance of all geological text-books, whether American 
or European, and it marks an epoch in scientific instruction." 

— The American Geologist. 

" The student is to be envied who can begin the study of this deeply 
interesting, fascinating subject with such an attractive help as this 
text-book." — Wooster Post-Graduate . 

"The Geology is admirably adapted for its purpose — that of a text- 
book." — Brooklyn Standard Union. 

" So admirable an exposition of the science as is found in this book 
must be welcomed both by instructors and students. The arrange- 
ment of facts is excellent, the presentation of theory intelligent and 
progressive, and the style exceedingly attractive." — N. V. Tribune. 



THE MACMILLAN COMPANY, 

66 FIFTH AVENUE, NEW YORK. 



